MODUL 1 ORGANIC CHEMISTRYCHAPTER 1 CHEMISTRY OF CARBONS

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TOPIC 1.1 BONDING OF THE CARBON ATOMS: THE SHAPES OF THE ETHANE, ETHENE, ETHYNE, AND BENZENE MOLECULES

LEARNING OBJECTIVES: 1. To explain the concept of hybridisation in the bonding of carbon atoms with reference specially to carbon atoms which have a valency of four and the types of hybridisation such as the following: sp linear, sp2 triangular, sp3 tetrahedral 2. To describe the formation of and bonds as exemplified by diagrams of the overlapping of orbitals in CH4, C2H4, C2H2, and C6H6 molecules 3. To explain the concept of delocalisation of electrons in benzene rings (aromaticity)

Hybridisation of 2s and 2p orbitals in carbon atoms: Each carbon atom has _____ valence electrons Valency of carbon atom is _____ Carbon could undergo 3 types of orbital hybridisation before it forms covalent bonds with another atom: _____________________________ Energy level diagrams to show the sp3, sp2 and sp hybridisation in carbon atom: Carbon atom (ground state) 2p Carbon atom (excited state) 2p sp3 Energy Energy Energy Energy 2s 2s Energy Carbon atom (sp3 hybridised) sp3 orbitals are formed The number of sp3 orbitals formed: ____ Energy of all sp3 orbitals: __________ Shape: __________ Bond angle: ______ e.g: 1s

1s

1s 2p

2 2s

Carbon atom (ground state) 2p

Carbon atom (excited state) 2p

Carbon atom (sp2 hybridised) 2p sp2

___ sp2 orbitals are formed __________ energy

Energy Energy

2s

Energy Energy

2s

Energy

1s Carbon atom (ground state) 2p 1s Energy Energy Energy Energy

1s Carbon atom (excited state) 2p

1s Carbon atom (sp hybridised) 2p sp

Shape: __________ Bond angle: ______ e.g: ___ sp orbitals are formed __________ energy

2s

2s

Energy

1s

1s

1s

Shape: __________ Bond angle: ______ e.g:

The Formation of and Bonds Two Types Of Covalent Bonds Formed (In Term Of Orbital Overlapping) sigma () bond orbitals overlapping : __________________________________________________ pi () bond orbitals overlapping : __________________________________________________ Tutorial: Based on how the following orbitals overlap, classify the following bonds as sigma () bond and pi () bond. sigma () bond pi () bond

3

CH4: Structural formula Molecular Shape Orbital hybridisation of Ca: _____________

H H Ca HOrbital overlapping diagram:

HShape: ______________________________ H-C-H bond angle: ____________________ The total number of bond: _____________ The total number of bond: _____________

C2H4: Structural formula H H Molecular Shape Orbital hybridisation of Ca: _____________ Orbital hybridisation of Cb: _____________ HCaH bond angle: ___________________ HCbH bond angle: ___________________ The total number of bond: _____________ The total number of bond: _____________

H

Ca

Cb

H

Orbital overlapping diagram:

C2H2: Structural formulaH Ca Cb H

Molecular Shape Orbital hybridisation of Ca: _____________ Orbital hybridisation of Cb: _____________ HCa Cb bond angle: _________________ The total number of bond: _____________ The total number of bond: _____________

Orbital overlapping diagram:

4

C6H6 : Structural formula H H

Molecular Shape

Orbital hybridisation of Ca,Cb,Cc,Cd,Ce & Cf: ____________________________________

Ca H Cf Ce

Cb Cc Cd H

HCaCb bond angle: __________________ CaCbH bond angle: __________________ HCdCe bond angle: __________________ CaCbCc bond angle: __________________ CdCeCf bond angle: __________________ The total number of bond: _____________ The total number of bond: _____________

H H Orbital overlapping diagram:

TOPIC 1.2

GENERAL, EMPIRICAL, MOLECULAR, AND STRUCTURAL FORMULAE FOR ORGANIC CHEMISTRY

LEARNING OBJECTIVES: 1. To explain the meaning of general, empirical, molecular, and structural formulae of organic compounds 2. To calculate empirical formulae and derive molecular formulae

Define: General formula Shows the molecular formula of a class of compounds, in which the number of atoms in the molecule is presented by x, y, n, etc. Empirical formula Shows the ____________________________ of the atoms present in the molecule. Molecular formula Shows the ________________________ of each type of atom present in the molecule. Structural formula Shows the ________________________ of each type of atom present in the molecule. Shows how the atoms are _________________ to each other. The structural formula can be represented in the form of: 5

Tutorial: Molecular Empirical Formula Formula

Condensed Structural Formula CH3CH2CH3

Displayed/ Expanded Structural Formula

Skeletal formula

CH3(CH2)4CH3

CH3CH2CH2OH

CH3CHOHCH(CH3)2

CH3CH2CH2Cl

Cl

CH3CH=CH2

CH3CH=C(CH3)CH(CH3)2

CH3COOCH(CH3)2

CH3CH2OCH2CH3

OH

O OH

CH3

CH3

H Cl O C C C OH H CH3 H H H C C C NH2 H CH3 H

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To determine the empirical formula of a compound, X Element Mass/ g Relative atomic mass, Ar Number of moles = Lowest Mole ratio Hence, the empirical formula of organic compound X = Given: Relative molecular mass = 62 Let the molecular formula of organic compound X = ( n = = C 1.00 O 1.33 H 0.25

)n =

Therefore, the molecular formula of organic compound X =

SUMMARY:Definitions of Molecular Formula, Empirical Formula & Structural FormulaMolecular formula The type of atoms in the molecule The number of each atom in the molecule The bonding between atoms in the molecule Empirical formula Structural formula

TOPIC 1.3

ISOMERISM: STRUCTURAL, GEOMETRIC, AND OPTICAL

LEARNING OBJECTIVES: 1. To interpret structural isomerism with reference to the ability of carbon atoms to link together with each other in a straight line and/or in branches 2. To explain geometric/cis-trans isomerism in alkenes in terms of restricted rotation due to bond/ C=C bonds 3. To explain the meaning of a chiral centre and how such a centre gives rise to optical isomerism 4. To identify chiral centres and/or cis-trans isomerism in a molecule of given structural formula 5. To determine the possible isomers for an organic compound of known molecular formula

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DEFINE: ISOMERISM :

ISOMERS :

TWO MAIN TYPES OF ISOMERISM Positional 1. Structural isomerism Chain Functional group cis Geometrical isomerism 2. Stereoisomerism Optical isomerism trans Mirror images (nonsuperimposable)

COMPARISON OF STRUCTURAL ISOMERISM AND STEREOISOMERISM: Type of isomers Structural isomers Stereoisomers STRUCTURAL ISOMERISM: Occurs when atoms are Occurs in compounds having the same different Three types of structural isomerism: Chain isomerism: Occurs when there are different arrangements of the Positional isomerism: Occurs when the same is placed at different . but Molecular formulae Structural formulae Functional group

Functional group isomerism: Occurs when there are different Chain isomerism: Example: C5H12

Pentane Example: C4H10

2-methylbutane

2,2 dimethylpropane

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Positional isomerism: Example: C4H8 But-1-ene But-2-ene

Example: C6H4Cl2

Example: C3H7OH

Functional group isomerism: Example: C2H6O

Ethanol (an alcohol)

dimethyl ether (an ether)

Different functional groups COMPARISON OF STRUCTURAL ISOMERISM: Type of structural Molecular Structural Carbon isomers formulae formulae chain Chain isomers Positional isomers Functional group isomers

Functional group

Position of Functional group

STEREOISOMERISM Occurs when compounds with same has different Two types of stereoisomerism: GEOMETRICAL ISOMERISM Meaning: Isomerism that occurs/ arises when .

Two conditions:

Example: But-2-ene:

H C H3C C

CH3 H

H C H3C C

H CH3

trans-but-2-ene 1,2-diiodocyclohexane:H I

cis-but-2-eneH I

H I

I H

cis-1,2-diiodocyclohexane 9

trans-1,2-diiodocyclohexane

CHCl=CHCl:

OPTICAL ISOMERISM Occurs in compounds with the same molecular formula and structural formula that Do not have a plane of symmetry. Have a chiral centre/ chiral carbon (this C* is attached to four different atoms/ groups) i.e: Important Terminology: Plane polarised light:

__________________________ __________________________ __________________________ Chiral carbon:

a y C* b x

__________________________ __________________________ __________________________ __________________________ Enantiomers:

The pair of optical isomers is called ____________________, they are ______________________ of one another, and are _______________________________.

Two conditions for optical activity:

__________________________ __________________________ __________________________ __________________________ __________________________

Example: 2-chloro-2-butanol: CH3

CH3CHClBr:

CH3 C2H5 C Cl HO, ie: is able to .

C Cl

C2H5 OH

Characteristic of enantiomers: Enantiomers are optically Are

superimposable. )

physical properties (except for Enantiomers have and chemical properties. Can be separated by a method called resolution. Initial After passing through optical isomer (+)-isomer/ dextrorotary isomer/ Disomer

Two types of enantiomers based on their optical activity (how they rotate plane-polarised light):

Plane-polarised light

()-isomer/ laevorotary isomer/ Lisomer

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How to remember the Lisomer and Disomer?

D A racemate/ racemic mixture:

LImportant Terminology: Racemate: __________________________ __________________________ __________________________ __________________________ __________________________

quantities of L Contains isomer and Disomer. Since the amounts of Lisomer and Disomer are the same, the two isomers cancel out each others light polarising effect. Hence, a racemate plane-polarised light.

TUTORIAL: 1. Draw all the structural formulae and give the IUPAC name of the isomers for a compound with a molecular formula of a. C3H7OH: [2] b. C3H4Cl2: [7]

2. Draw and name all the stereoisomers for compound Y with a molecular formula of CH3CH=CHCl.

SUMMARY: ISOMERISMTypes of Isomers Chain isomers Structural Positional isomers isomers Functional group isomers Stereoisomers: :

Molecular Structural Spatial formulae formulae arrangement

m.p. & b.p.

Chemical Chemical Optical properties reactivity activity

Enantiomers

D: L:

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TOPIC 1.4 TOPIC 1.5

CLASSIFICATION BASED ON FUNCTIONAL GROUPS (GENERAL FORMULA) NOMENCLATURE AND STRUCTURAL FORMULAE FOR EACH FUNCTIONAL/RADICAL GROUP (REFER TO THEIR TRIVIAL NAMES)

LEARNING OBJECTIVES: 1. To describe the classification of organic compounds by functional groups and the nomenclature of classes of organic compounds according to the IUPAC system of the following classes of compounds: a. alkanes, alkenes b. haloalkanes c. alcohols (including primary, secondary and tertiary) and phenols d. aldehydes and ketones e. carboxylic acids and esters f. primary amines, amides, and amino acids Homologous series 1 2 3 4 5 6 7 8 9 10 11 12 Alkane Alkene Haloalkane Alcohol Phenol Aldehyde Ketone Carboxylic acid Ester Primary amine Amide Amino acid Functional group Structural formula Nomenclature

General formula

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NOMENCLATURE: Prefix Indicates the number of carbon atoms in the longest continuous chain.Number of C atoms 1 2 3 4 5 6 7 8 9 10 Prefix MethEthPropButPentHexHeptOctNonDecNumber of Prefix C atoms 11 Undec12 Dedec13 Tridec14 Tetradec15 Pentadec16 Hexadec17 Heptadec18 Octadec19 Nonadec20 Eicos-

Suffix Represents the homologous series of the compound. Homologous series Alkane Alkene Haloalkane Alcohol Aldehyde Ketone Carboxylic acid Primary amine Amide Suffix -ane -ene Halo- -ane -ol -al -one -anoic acid -anamine -anamide

IUPAC Nomenclature of organic compounds: 1. Find the longest chain of carbons in the molecule. The number of carbons in the longest chain becomes the parent name (refer to the above table) 2. After finding the parent chain, you number the parent chain starting with the end nearest to the first substituent (a substituent is any fragment that juts off the main chain). 3. Next, determine the names of all substituents. Substituents are named as if the piece were a separate molecule, except that the suffix of -yl is used rather than -ane. E.g.: Alkyl, R Structural formula Alkyl, R Structural formula methyl CH3 ethyl C2H5 / CH2CH3 isobutyl CH3CH2CHCH3

propyl CH2CH2CH3 phenyl C6H5

isopropyl CH3 CHCH3

butyl CH2CH2CH2CH3 benzyl C6H5CH2

CH2CH (CH3)2

Or

CH2

4. Put the substituents in alphabetical order (ie. ethyl before methyl) in front of the parent name. 5. Next, identify the positions of all substituents in the name by placing the carbon number where the substituent attaches to the parent chain in front of it. For identical substituents, use the prefix di-, tri- or tetra-. 6. Separate numbers using commas. 7. Separate numbers from letters by hyphens.

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TUTORIAL: For each of the following molecules, identify its homologous series and name the compound. (a) (b) (c) (d) (e) (f) C6H5CH2 = CH2 CH2CH2CH(OH)CH3 C6H5CH2OH C6H5OH C6H5COOH CH3CH2COOH (g) (h) (i) (j) (k) (l) HCOOCH2CH3 CH3COOC6H5 CH3CH2CH2NH2 CH3CH(NH2)CH2CH3 CH3CH2CH2Br CH3CH2CH2CONH2

TOPIC 1.6

NUCLEOPHILE AND ELECTROPHILE

LEARNING OBJECTIVES: 1. To define the terms nucleophile and electrophile 2. To explain the meaning of Lewis acids and bases in terms of charge/electron density 3. To explain why many organic compounds containing oxygen/nitrogen which have lone pair electrons (as Lewis bases) form bonds with electron acceptors (as Lewis acids) 4. To explain how nucleophiles such as OH, NH3, H2O, Br, I and carbonion have Lewis base properties, whereas electrophiles such as H+, NO2+, Br2, AlCl3, ZnCl2, FeBr3, BF3, and carbonium ions have Lewis acid properties TUTORIAL: Classify each of the following species as electrophile or nucleophile and explain your choice. (a) CN (i) OH (q) ZnCl2 (b) H2O (c) AlCl3 (d) Br (e) H+ (f) CH2=CH2 (g) NO2+ (h) HCl Comparison: Species Electron density Charge (for ionic species) Presence of lone e pairs Electron acceptor/ donor Lewis acid/ base Oxidising/ Reducing Property 14 Nucleophile, Electrophile, E+ (j) NH3 (k) I (l) FeCl3 (m) CH3CH2+ (n) CH3CH2NH2 (o) Br2 (p) CH3COO (r) BF3 (s) CH3CH2NH (t) FeBr3 (u) HSO4 (v) HSO3+ (w) P(CH3) 3 (x) CH3CH2O Lewis base : Lewis acid : Define: Nucleophile : Electrophile :

Reaction between nucleophile (Lewis base) and electrophile (Lewis acid): Identify the following reactants as electrophiles or nucleophiles. 1. 2. 3. 4. 5. CH3Cl + OH CH3OH + Cl CH3CHO + Cl CH3CHClO FeCl3 + Cl2 FeCl4 + Cl+ CH2=CH2 + Br2 CH2BrCH2Br CH3CH2NH2 + HCl CH3CH2NH3+ + Cl

Classification of free radicals, electrophiles and nucleophiles: Free radicals: H H H

R R C R

H C H Electrophiles: H

R C H

R

C R

H R C H

H R C R

R R C R

H C H Nucleophiles:

Stability of free radicals, electrophiles and nucleophiles: R : Electron donating group Free radicals: Electrophiles: Nucleophiles: CH3 CH3+ CH3 1 1 1 2 2 2 3 3 3

Important facts: FR, E+ or appears as intermediate in rxn mechanisms. If the intermediate is stable, the reaction occurs ________________ If the intermediate is unstable, the reaction occurs ________________ Species with higher charge density _________ stable Species with lower charge density _________ stable

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6 Common Organic Reactions: Organic reactions AdditionEA: Alkene + Br2/ H2O/ HX NA: Aldehyde/ ketone + HX

SubstitutionFRS: Alkane + Halogen ES: Benzene + NO2+/ Cl+/ Br+/ HSO3+/ R+ NS: ROH / RX + Nu

EliminationDehydration (H2O): ROH Dehydrohalogenation (HX): RX

INDUCTIVE EFFECT ON THE CHEMICAL PROPERTIES AND REACTIONS OF THE FUNCTIONAL GROUP Homologous series 1 2 3 4 Alkane Functional group Chemical Reaction Succeptible to attack of ... Type of reaction Intermediate

Alkene Haloalkane Alcohol

5 6 7 8

Aldehyde Ketone Carboxylic acid Primary amine

TUTORIAL: Classify each of the following organic reactions as elimination/ substitution/ addition: (a) CH2=CH2 + H2 CH3CH3 (b) (c) (d) (e) (f) CH3CH2CH3 CH3CH=CH2 + H2 CH2=CH2 + Br2 CH2BrCH2Br CH3CH2CH3 + Br2 CH3CH2CH2Br + HBr CH2=CH2 + HOBr CH2BrCH2OH CH3CH2Br + CN CH3CH2CN + Br 16

(g) (h) (i) (j) (k) (l)

CH3OH + NH2 CH3NH2 + OH CH4 + Cl2 CH3Cl + HCl CH3CH2CN + 2H2 CH3CH2CH2NH2 CH3COCl + NH2 CH3CONH2 + Cl CH3COCl + CH3O CH3COOCH3 + Cl CH3CH2OH CH2=CH2 + H2O

TOPIC 1.7

STRUCTURE AND ITS EFFECT ON

(A) Physical Properties; e.g. Boiling Point, Melting Point, And Solubility In Water (B) Acidity And Basicity The Effect Of The Structure And Delocalisation Of Electrons On The Relative Acid Or Base Strength, i.e. Proton Donors Or Acceptors, In Ethylamine, Phenylamine, Ethanol, Phenol, And Chlorine-Substituted Ethanoic Acids LEARNING OBJECTIVES:1. To describe the relationship between the size of molecules in the homologous series and the melting and boiling points 2. To explain the attractive forces between molecules (van der Waals forces and hydrogen bonding) 3. To explain induction effect which can determine the properties and reactions of functional groups 4. To explain how most functional groups such as NO2, CN, COOH, COOR, >C=O, SO3H, X (halogen), OH, OR, NH2, C6H5 are electron acceptors whereas functional groups such as CH3, R (alkyl or aryl) are electron donors 5. To explain how the concept of induction can account for the differences in acidity between CH3COOH, ClCH2COOH, Cl2CHCOOH, and Cl3CCOOH; between ClCH2CH2CH2COOH and CH3CH2CHClCOOH 6. To use the concept of delocalisation of electrons to explain the differences in acidity between ethanol and phenol, as well as the differences in basicity between CH3NH2 and C6H5NH2

STRUCTURE AND ITS EFFECT ON BOILING POINT AND MELTING POINT:

POLAR MOLECULES (H2O, ROH, RCOOH, NH3, RNH2) Hydrogen bonds

POLAR MOLECULESNON-POLAR MOLECULES Van der Waals attractive forces (induced dipole - induced dipole attractive forces) 17 Van der Waals attractive forces (permanent dipole - permanent dipole attractive forces)

1. NON-POLAR MOLECULES: with different molecular mass: E.g.: CH4 < C3H8: Because: Molecular size:

Bigger molecule contains more

.

The strength of intermolecular attractive forces (Van der Waals):

with approximately the same molecular mass/ molecular formula: The shape of molecules that are extensively branched : The shape of linear molecules : E.g.: Boiling point CH3CH2CH2CH2CH3 > CH3CH(CH3)CH2CH3 > CH3CH(CH3)2CH3 : Because: Degree of branching: Contact surface area between molecules: The strength of intermolecular attractive forces (Van der Waals): E.g.: Melting point CH3CH2CH2CH2CH3 < CH3CH(CH3)CH2CH3 < CH3CH(CH3)2CH3 : Because: Degree of branching: Arrangement and packing in solid state: (more compact) > > (less compact) The strength of intermolecular attractive forces (Van der Waals):

2. POLAR AND NON-POLAR MOLECULES: E.g.: C6H10 < C3H7Cl : Because: Type of intermolecular attractive forces in: C6H10 : C3H7Cl : The strength of i.m.f.: Covalent molecules Because: More energy is required to break the strong Less energy is needed to overcome the weak in covalent compound.

in ionic compound

3. COMPOUNDS WITH INTERMOLECULAR HYDROGEN BONDS: H2O, NH3, CH3NH2 , CH3COOH, CH3OH Intermolecular forces:TUTORIAL: Melting and boiling points

The strength of intermolecular forces: > >

1.

Different types of intermolecular forces exist in organic compounds. a. Name these intermolecular forces. b. Indicate the type(s) of intermolecular forces present in each of the compounds below. (i) CH3CH2CH2CH2OH (ii) CH3CH2COOCH2CH3 (iii) CH3CH2CH2CH2CONH2 (v) CH3CH2CH2COOH (vii) (ix) (iv) CH3CH2CH2CH2CHO (vi) CH3CH2CH2COCH3 (viii)

CH3

COCl

Cl

(x) CH3CH2CH2NH2 (xii) CH3CH2OCH2CH3

(xi) CH3CH2CH2CH2CN 2.

Which of the following compounds have hydrogen bonding? A (CH3)3N B CH3OH C CH3OCH3 D CHI3 E

COOH

3.

Arrange the following compounds in increasing order of boiling points. CH3CH2CH2OH, CH3CH2CH2CH3, HOCH2CH2OH, CH3COCH3

4. The boiling points of isomeric pentane are shown below: I II Isomer Boiling point / C 36 28

III 10

Draw the structural formulae of the three isomers and explain the difference in their boiling points.

19

5.

Arrange the following compounds in decreasing order of volatility. a. b. c. CH3CH2NHCH3, (CH3)3N, CH3CH2CH2NH2 (CH3)3COH, CH3CH2CH2CH2OH, CH3CHCH2CH3

OH HCOOCH3, CH3COOH, CH3CH2CH2OH

6.

What are the strongest attractive forces that must be overcome to: a. b. boil CHCl3 vapourise

c.

melt

COOH

STRUCTURE AND ITS EFFECT ON SOLUBILITY IN WATER 1. COMPOUNDS WITH INTERMOLECULAR HYDROGEN BONDS (ALCOHOL, CARBOXYLIC ACID, AMINE & AMIDE):

The ultimate rule: Like-Dissolves-Like H2O, NH3, CH3NH2 , CH3COOH, CH3OH, CH3CONH2 Intermolecular attractive forces: These molecules are miscible with water. Because: E.g.: H2O and NH3 , H2O and CH3NH2, H2O and CH3COOH, H2O and CH3OH Condition: Total number of carbon atoms in the molecule must not exceed Because:

2. H2O & ORGANIC COMPOUNDS THAT ARE NOT MENTIONED ABOVE: Non-polar compounds: Alkane, alkene, benzene, Intermolecular forces in non-polar compounds: Insoluble/ Immiscible in water Polar compounds: haloalkane Intermolecular forces in polar compounds: Insoluble/ Immiscible in water Compounds that contains O atom: E.g.: Carbonyl compound (Functional group: C=O group, i.e: aldehyde and ketone), ester (RCOOR) amide (RCOONH2), ether (ROR) Total number of carbon atoms in the molecule 4 (n = 1, 2, 3 or 4): Total number of carbon atoms in the molecule > 4 (n = 4, 5, 6, ...): 20

TUTORIAL: Solubility 1. Which of the following compounds are soluble in water? (a) CH3CH2CH2CH2CH3 (b) CH3CHO (c) CH3COCH3 (d) (n) CH3CH2OCH2CH3 (o) CH3CH2CH2NH2 (p) CH3CH2CH2CH2OH (q)

OH

COCl

(e) CH3CH2OH (f) CH3COOCH3 (g) CH3CH2CH2COOH (h)

(r) CH3CH2COOCH2CH3 (s) HCOOCH3 (t) CH3CH2Cl (u)

Cl

CH3

(i) CH3CH2CH2CH2CONH2 (j) CH3CH2CH2CH2CN (k) CH3CH2COOCH2CH3 (l) CH3CH2CH2CH2CHO (m) 2.

(v) CH3CH2CH2COCH3 (w) CH3CH2NH2 (x) C2H5OC2H5 (y) CH3CONH2 (z)

COOH

CHO

Arrange the following compounds in increasing order of solubility in water.

OH3.

CH3CH2CH2OH

HOCH2CH2OH

Which of the following compounds is insoluble in benzene? NH2 CH3CH2NH 2 A B C CH3CHCOOH CH3CONH2 D E CH3CH2CH2CN F

CH3CH2COOH CH3CH2CH2Br

STRUCTURE AND ITS EFFECT ON ACIDITY & BASICITY INDUCTIVE EFFECT MESOMERIC EFFECT EFFECT OF STRUCTURE

INDUCTIVE EFFECT: An electronic effect transmitted by successive polarisation of the bonds within a molecule. POSITIVE INDUCTION EFFECT/ ELECTRON DONATING EFFECT/ ELECTRON REPELLING EFFECT: Electro species e.g.: species e.g.: 21 NEGATIVE INDUCTION EFFECT/ ELECTRON WITHDRAWING EFFECT: Electro

RESONANCE EFFECT/ MESOMERIC EFFECT: Electron distribution/ delocalisation. Usually occurs in the following structures:

STRUCTURE AND ITS EFFECT ON ACIDITY The strength of an acid is measured by the extent it donates a proton to water. IUPAC naming E.g.: (conjugate bases): HCOOH + H2O CH3COOH + H2O CH2ClCOOH + H2O CHCl2COOH + H2O CCl3COOH + H2O Origin (Acid) Carboxylic acid Alcohol -suffix

-oate

-oxide

OH + H2O

CH3OH + H2O CH3CH2OH + H2O By referring to the acid dissociation equations in the above, the more an acid ionise in water (i.e. the further the equilibrium position lies to the RHS), the conjugate base is more the the [H3O+] . the acid.Recall concept of the stability of Nucleophile: 1 > 2 > 3 An anion is stabilised by lowering its charge density. Positive inductive (electron donating) effect by alkyl groups destabilises the anion Because negative charge density on anion ________________ Negative inductive (electron withdrawing) effect by electronegative species stabilises the anion Because negative charge density on anion ________________

Example 1: Acidic strength: H2O > CH3OH _______________ inductive effect of CH3 Electron density on oxygen atom in CH3OH: Stability of CH3O ___________________ Extent of dissociation in water: Example 2: Acidic strength: CH3OH > CH3CH2OH __________________ inductive effect of CH2CH3 is stronger Electron density on oxygen atom: Stability of conjugate base: Extent of dissociation in water: 22

Example 3: Acidic strength: CH3CH2CH2OH > C(CH3)3OH ________________ inductive effect of three CH3 is stronger than one CH2CH2CH2CH3 Electron density on oxygen atom: Stability of conjugate base: Example 4: Acidic strength:

OH > ROH (aliphatic alcohol)

_______________ inductive effect of R Stability of RO ____________________ In

O-, lone electron pair on oxygen atom is

into

benzene ring _______________ stabilisation of Electron density on oxygen atom: Stability of conjugate base: Extent of dissociation in water: Example 5: Acidic strength: CH3COOH > CH3CH2OH

O-

______________ inductive effect of CH3CH2 Stability of CH3CH2O ___________ _______________ effect of C=O Stability of CH3COO ___________ Electron density on oxygen atom: Stability of conjugate base: Extent of dissociation in water: Example 6: Acidic strength: CH3COOH > The presence of benzene ring in stabilisation of In both Diagram:

OHOH and C=O causes _________________

O- and CH3COO O- and CH3COO , lone electron pair of oxygen atom is

But, oxygen atom is __________ electronegative than carbon atom. Hence, resonance effect (delocalisation of negative charge) of C=O is Electron density on oxygen atom: Stability of conjugate base: Extent of dissociation in water: 23 than benzene ring.

Example 7: Acidic strength: Cl

OH >

OHO- _____

______________ inductive effect of chlorine atom Stability of Cl Electron density on oxygen atom: Stability of conjugate base: Extent of dissociation in water:

Cl Example 8: Acidic strength:

OH

> Cl

OH

Cl Chlorine atom is nearer to OH in chlorine atom is Electron density on oxygen atom: Stability of conjugate base: Extent of dissociation in water:

OH

______________ inductive effect of

Cl Example 9: Acidic strength:

Cl

OH

> Cl

OHCl

______________ inductive effect of 2 chlorine atoms in Electron density on oxygen atom: Stability of conjugate base: Extent of dissociation in water: Example 10: Acidic strength: NO2

Cl

OH

is

OH > Cl

OHthan chlorine atom

NO2 is __________ electronegative than chlorine atom ______________ inductive effect of NO2 is Electron density on oxygen atom: Stability of conjugate base: Extent of dissociation in water: 24

Example 11: Acidic strength: CH3CHClCOOH > CH3CH2COOH ____________ inductive effect of chlorine atom Stability of CH3CHClCOO ________ Stability of conjugate base: Extent of dissociation in water: Example 12: Acidic strength: CH3CHFCOOH > CH3CHClCOOH > CH3CHBrCOOH Electronegativity of substituents: __________ inductive effect of substituents: Stability of conjugate base: Extent of dissociation in water: Example 13: Acidic strength: CH3CCl2COOH > CH3CHClCOOH __________ inductive effect of two chlorine atoms is _______________________ than one chlorine atom. Stability of conj. base: Extent of dissociation in water: Important fact: Acid 1 is stronger than Acid 2: Ka1 ________ Ka2 pKa1 ________ pKa2 [H3O+]: ______________ pH: _________________

SUMMARY: Acid strength: RCOOH

>

OH

>

H2O

>

ROH

EFFECT OF INDUCTIVE EFFECT AND MESOMERIC EFFECT ON ACID STRENGTH ELECTRON ACCEPTORS e.g.: X, NO2, C6H5 exerts negative inductive effect (I effect) / electron withdrawing effect Increases acid strength CH2ClCOOH is a stronger acid than CH3COOH ELECTRON DONORS e.g.: R exerts positive inductive effect (+I effect) / electron repelling effect Decreases acid strength CH3COOH is a weaker acid than HCOOH CH3OH is a weaker acid than H2O MESOMERIC EFFECT Delocalisation of electrons in e.g.: (C6H5) Increases acid strength C6H5OH is a stronger acid than CH3OH CH3COOH is a stronger acid than CH3OH ,

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STRUCTURE AND ITS EFFECT ON BASICITY (CONSIDERING LEWIS DEFINITION OF BASES) The strength of a Lewis base is measured by the availability its lone pair of electrons on its nitrogen atom to be donated to a proton. E.g.: NH3 + H2O CH3NH2 + H2O CH3CH2NH2 + H2O (CH3)2NH + H2O

NH2 + H2ORecall concept of the stability of Electrophile: 1 < 2 < 3 A cation is stabilised by lowering its charge density. Positive inductive (electron donating) effect by alkyl groups stabilises the cation Because its positive charge density __________ Negative inductive (electron withdrawing) effect by electronegative species destabilises the cation Because its positive charge density __________ Important fact: Base 1 is stronger than Base 2: Kb1 ________ Kb2 pKb1 ________ pKb2 [OH]: ________________ pOH: _________________ pH: ___________________

By referring to the equations in the above, the more a base ionise in water (i.e. the further the equilibrium position lies to the RHS), the conjugate acid is more the the [OH] . the base.

Example 1: Basic strength: CH3NH2 > NH3 _______________ inductive effect of CH3 Electron density on nitrogen atom in CH3NH2 : ____________ The ease in donating lone electron pair to a proton: ___________________________________________________ Stability of conjugate acid: Example 2: Basic strength: CH3CH2NH2 > CH3NH2 .

__________________ inductive effect of CH2CH3 is stronger than CH3 Electron density on nitrogen atom: The ease in donating lone electron pair to a proton: Stability of conjugate acid : Example 3: Basic strength: CH(CH3)2NH > CH3CH2CH2NH2 ________________ inductive effect of two CH3 is stronger than one CH2CH2CH3 Electron density on nitrogen atom: The ease in donating lone electron pair to a proton: Stability of conjugate acid : 26

Example 4: Basic strength: CH3CH2CH2NH2 > CH3CHClCH2NH2 ________ inductive effect of chlorine atom Stability of CH3CHClCH2NH3+ _______ Electron density on nitrogen atom: The ease in donating lone electron pair to a proton: Stability of conjugate acid : Example 5: Basic strength: RNH2 (aliphatic amine) >

NH2

_______________ inductive effect of R Stability of RNH3+ ____________________ effect of benzene ring lone electron pair on nitrogen atom is delocalised into benzene ring stability of Electron density on nitrogen atom: The ease in donating lone electron pair to a proton: Stability of conjugate acid :

NH3+

Example 6: Basic strength:

NH2 > Cl

NH2 NH3+ _______

________ inductive effect of chlorine atom Stability of Cl Electron density on nitrogen atom: The ease in donating lone electron pair to a proton: Stability of conjugate acid :

Example 7: Basic strength: CH3

NH2 >

NH2

___________ inductive effect of CH3 Stability of CH3 Electron density on nitrogen atom: The ease in donating lone electron pair to a proton: Stability of conjugate acid :

NH3+ _________

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TUTORIAL: 21: 3. Acidity of carboxylic acids 3.1 a. Arrange the following compounds in increasing of pKa values:

b.

Match the acids in each set with the pKa values given within brackets.

SUMMARY: RNH2 > NH3 >NH2

EFFECT OF INDUCTIVE EFFECT & MESOMERIC EFFECT ON BASIC STRENGTH ELECTRON ACCEPTORS e.g.: X, NO2, C6H5 exerts negative inductive effect (I effect) / electron withdrawing effect ELECTRON DONORS e.g.: R exerts positive inductive effect (+I effect) / electron repelling effect MESOMERIC EFFECT Delocalisation of electrons in e.g.: (C6H5)

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Decreases basic strength CH2ClNH2 is a weaker base than CH3NH2

Increases basic strength CH3NH2 is a stronger base than NH3

Decreases basic strength C6H5NH2 is a weaker base than CH3NH2

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Must know for the MCAT

Stereochemistry

Stereochemistry involves the study of the relative spatial arrangement of atoms within molecules.

Chiral

The term chiral is used to describe an object that is non-superimposable on its mirror image.

Cis-trans isomerism

Cis-trans isomerism is a form of stereoisomerism describing the orientation of functional groups typically around double bonds which cannot rotate.

Steric effects

Steric effects arise from the fact that if atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds.

Enantiomer

Enantiomers are stereoisomers that are nonsuperimposable complete mirror images of each other.

Racemic

A racemic mixture is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule.

Diastereomer

Diastereomers are stereoisomers that are not enantiomers.

Optical activity

Optical rotation or optical activity is the rotation of linearly polarized light as it travels through certain materials.

Meso compound

A meso compound is a chemical compound with molecules that contain 2 or more stereocenters but which is optically achiral because it contains an internal plane of symmetry.

Newman projection

A Newman projection visualizes chemical conformations of a carbon-carbon chemical bond from front to back, with the front carbon represented by a dot and the back carbon as a circle.

Staggered conformation

A staggered conformation is a chemical conformation that exists in any open chain single chemical bond connecting two sp3 hybridised atoms as a conformational energy minimum.

Eclipsed conformation

An eclipsed conformation is a chemical conformation that exists in any open chain single chemical bond connecting two sp3 hybridised atoms as a conformational energy maximum.

Stereocenter

A stereocenter is any atom in a molecule bearing groups such that an interchanging of any two groups leads to a stereoisomer.

Conformational isomerism

Conformational isomerism is a form of stereoisomerism involving molecules with the same structural formula existing as different conformers due to atoms rotating about a bond.

Van der Waals strain

Van der Waals strain results from van der Waals repulsion when two substituents in a molecule approach each other with a distance less than the sum of their van der Waals radii.

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Cyclohexane

Cyclohexane is a cycloalkane containing 6 carbons and 12 hydrogens, which has the lowest angle and torsional strain of all the cycloalkanes.

Angle strain

The presence of angle strain in a molecule indicates that in a specific chemical conformation bond angles are deviating from the ideal bond angles required to achieve maximum bond strength.

Should know for the MCAT

Ring strain

Ring strain is an organic chemistry term that describes the destabilization of a cyclic molecule-such as a cycloalkane-due to the non-favorable high energy spatial orientations of its atoms.

Chiral resolution

Chiral resolution in stereochemistry is a process for the separation of racemic compounds into their enantiomers.

Prochiral

Prochiral molecules can be converted from achiral to chiral in a single step.

Hyperconjugation

Hyperconjugation is the stabilizing interaction that results from the interaction of the electrons in a sigma bond with an adjacent empty or partially filled non-bonding p-orbital or antibonding pi orbital leading to an extended molecular orbital that increases the stability of the system.

Anomeric effect

The anomeric effect or Edward-Lemieux effect describes the tendency of heteroatomic substituents adjacent to a heteroatom within a cyclohexane ring to prefer the axial orientation instead of the expected, lesshindered equatorial orientation.

Enantiomeric excess

Enantiomeric excess exists where one enantiomer is present more than the other in a chemical substance.

May appear in context in MCAT passages - advanced terminology

Baeyer strain theory

Baeyer strain theory explains specific behaviour of chemical compounds in terms of bond angle strain.

Enantiomer selfdisproportionation

Enantiomer self-disproportionation is a process describing the separation of a non-racemic mixture of enantiomers in an enantioenriched fraction and a more racemic fraction.

Homochirality

Homochirality is a term used to refer to a group of molecules that possess the same sense of chirality with similar groups are arranged in the same way around a central atom.

Chiral pool synthesis

Chiral pool synthesis is a strategy that aims to improve the efficiency of chiral synthesis by beginning from a stock of readily available enantiopure substances.

Asymmetric

Asymmetric induction describes the preferential formation in a chemical reaction of one enantiomer or

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induction

diastereoisomer over the other as a result of the influence of a chiral feature present in the substrate, reagent, catalyst or environment.

Chiral auxiliary

A chiral auxiliary is a chemical compound or unit that is temporarily incorporated into an organic synthesis so that it can be carried out asymmetrically with the selective formation of one of two enantiomers.

Circular dichroism

Circular dichroism is a form of spectroscopy based on the differential absorption of left- and right-handed circularly polarized light.

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