1
Organic Chemistry
By Dr.Ahmed AbdoulAmier Hussain Al-Amiery
2
Electronic configuration of Carbon
C 1s2 2s2 2p2 • Covalent bonds: sharing of electrons between atoms • Carbon: can accept 4 electrons from other atoms • i.e. Carbon is tetravalent (valency = 4)
Ethane: a gas (b.p. ~ -100oC Empircal formula (elemental combustion analysis): CH3 Measure molecular weight (e.g. by mass spectrometry): 30.070 g mol-1, i.e (CH3)n n = 2 Implies molecular formula = C2H6 Molecular formula: gives the identity and number of different atoms comprising a molecule Ethane: molecular formula = C2H6 Valency: Carbon 4 Hydrogen 1 Combining this information, can propose
structural formula for ethane
• Each line represents a single covalent bond • i.e. one shared pair of electrons
• Structural formulae present information on atom-to-atom connectivity
C CH
HH
H
HH
C C
H
H
HH
H
H
3
• However, is an inadequate represention of some aspects of
the molecule
• Suggests molecule is planar • Suggests different types of hydrogen
Experimental evidence shows: • Ethane molecules not planar • All the hydrogens are equivalent • 3 Dimensional shape of the molecule has tetrahedral
carbons • Angle formed by any two bonds to any atom = ~ 109.5o
Angle between any two bonds at a Carbon atom = 109.5o
109.5°
C C
H
H
H
H
H
H
109.5o
C C
H
H
H
H
H
H109.5o
4
Electronic configuration of Carbon C 1s2 2s2 2p2 Hydrogen H 1s1
• However, know that the geometry of the Carbons in ethane is tetrahedral
• Cannot array py and pz orbitals to give tetrahedral geometry
• Need a modified set of atomic orbitals - hybridisation
C CH
HH
HH
HEthane
Orbitals available for covalent bonding?
H 1s(1 e ) C 2py
(vacant)
C 2pz
(vacant)
5
Bonding in ethane Atomic orbitals available: 2 Carbons, both contributing 4 sp3
hybridised orbitals 6 Hydrogens, each contributing an s orbital
Total atomic orbitals = 14 Combine to give 14 molecular orbitals 7 Bonding molecular orbitals; 7 anti-bonding molecular orbitals Electrons available to occupy molecular orbitals One for each sp3 orbital on Carbon; one for each s orbital on Hydrogen Just enough to fully occupy the bonding molecular orbitals Anti-bonding molecular orbitals not occupied Ethane: molecular orbital diagram
δ molecular orbitals: symmetrical about the bond axis Four sp3 hybridised orbitals can be arrayed to give tetrahedral geometry,
σCH
σ*CH
σCC
σ*CC
Energy C C
H
H
H
H
H
H
6
sp3 hybridised orbitals from two Carbon atoms can overlap to form a Carbon-Carbon s bond
An sp3 orbital extends mainly in one direction from the nucleus and forms bonds with other atoms in that direction.
7
This represents a particular orientation of the C-H bonds on adjacent Carbons
Newman projection
Staggered conformation: Minimum energy conformation (least crowded possible conformation) Eclipsed conformation: Maximum energy conformation (most crowded possible conformation)
• Eclipsed conformation experiences steric hindrance
H
H H
H
H
H
H
H H
H
H
H
H
HH
H
HH
8
• Unfavourable interaction between groups which are close together in space
• These unfavourable interactions absent in the staggered conformation
• Hence, the staggered conformation is lower in energy • Energy difference between eclipsed and staggered
conformations of ethane = 12 kJ mol-1 • Each C-H eclipsing interaction contributes 4 kJ mol-1 of
torsional strain energy
Conformations: different orientations of molecules arising from rotations about C-C s bonds Consider one full rotation about the C-C bond in ethane Start at φ = 0° (eclipsed conformation)
Staggered conformation strain energy 0 kJ mol-1 Eclipsed conformation strain energy 12 kJ mol-1 Identical to that at φ = 0° Hence, in one full rotation about the C-C bond
• Pass through three equivalent eclipsed conformations (energy maxima)
H
HH
H
HH
4 kJ mol-1
4 kJ mol-14 kJ mol-1
9
• Pass through three equivalent staggered conformations (energy minima)
• Pass through an infinite number of other conformations
10
Some points concerning this series of alkanes 1. Series is generated by repeatedly adding ‘CH2’ to the revious member of the series. A series generated in this manner is known as an homologous series
2. Nomenclature (naming): Names all share a common suffix, i.e.’ …ane’ The suffix ‘…ane’ indicates that the compound is an alkane The prefix indicates the number of carbons in the compound
3. Representation and conformation :
C CH
HH
H
HCH
HCH
HH
Butane(full structural formula)
11
• Structural formulae: give information on atom-to-atom connectivity
•
Butane CH3-CH2-CH2-CH3
One full 360° rotation about the central C-C of butane: Pass through three staggered and three eclipsed conformations
H
HH
H
CH3
H HH
H
H
CH3
H
12
13
14
Alkenes Degree of Unsaturation n Relates molecular formula to possible structures n Degree of unsaturation: number of multiple bonds or rings n Formula for saturated a acyclic compound is CnH2n+2 n Each ring or multiple bond replaces 2 H's
Example: C6H10 n Saturated is C6H14
n Therefore 4 H's are not present n This has two degrees of unsaturation
n Two double bonds? n or triple bond? n or two rings n or ring and double bond
Degree of Unsaturation With Other Elements n Organohalogens (X: F, Cl, Br, I)
n Halogen replaces hydrogen n C4H6Br2 and C4H8 have one degree of unsaturation n Oxygen atoms - if connected by single bonds
CH3 CCH3
CH3
CH2 CCH3
HCH3
2,2,4-Trimethylpentane
CH3 CH2 CCH3
CH2
CH2 CCH3
CH3
CH3 H 1
23456
2,4-dimethyl-4-ethylhexane
H3CC
CC
CCH3
H H
H H
H H
H H
15
n These don't affect the total count of H's
Degree of Unsaturation and Variation n Compounds with the same degree of unsaturation can have
many things in common and still be very different
Summary - Degree of Unsaturation n Count pairs of H's below CnH2n+2 n Add number of halogens to number of H's (X equivalent to
H) n Don't count oxygens (oxygen links H) n Subtract N's - they have two connections
Naming of Alkenes n Find longest continuous carbon chain for root n Number carbons in chain so that double bond carbons have
lowest possible numbers n Rings have “cyclo” prefix
16
Many Alkenes Are Known by Common Names: n Ethylene = ethene n Propylene = propene n Isobutylene = 2-methylpropene n Isoprene = 2-methyl-1,3-butadiene
Alkene Nomenclature
17
Common names
Isomers and stability
Cis-Trans Isomerism in Alkenes n The presence of a carbon-carbon double can create two
possible structures n cis isomer - two similar groups on same side of the
double bond n trans isomer similar groups on opposite sides
n Each carbon must have two different groups for these isomers to occur
18
Naming stereoisomers: The E-Z system (vs. cis- and trans-)
Electronic Structure of Alkenes n Carbon atoms in a double bond are sp2-hybridized
n Three equivalent orbitals at 120º separation in plane n Fourth orbital is atomic p orbital
n Combination of electrons in two sp2 orbitals of two atoms forms σ bond between them
n Additive interaction of p orbitals creates a π bonding orbital n Subtractive interaction creates a π anti-bonding
orbital n Occupied π orbital prevents rotation about σ-bond n Rotation prevented by π bond - high barrier, about 268
kJ/mole in ethylene
19
Rotation of π Bond Is Prohibitive
n This prevents rotation about a carbon-carbon double bond
(unlike a carbon-carbon single bond). n Creates possible alternative structures
Alkene Stability: n Cis alkenes are less stable than trans alkenes n Compare heat given off on hydrogenation: ∆Ho n Less stable isomer is higher in energy
n And gives off more heat n tetrasubstituted > trisubstituted > disubstituted >
monosusbtituted n hyperconjugation stabilizes alkyl
Comparing Stabilities of Alkenes n Evaluate heat given off when C=C is converted to C-C n More stable alkene gives off less heat
n Trans butene generates 5 kJ less heat than cis-butene
20
Preparation of Alkenes
Dehydration of Alkenes
Regioselectivity in alcohol dehydration: Zaitsev’s Rule
21
Stereoselectivity in ROH dehydration
E1 and E2 Mechanisms of alcohol dehydration n Use what you know to predict the mechanism for this
reaction
n Resembles ROH + HX n Both promoted by acids n Reactivity rate 3º > 2 º > 1 º
Same thing:
22
Rearrangements: methyl shift:
Rearrangements: hydride shift
Dehydrohalogenation of R-X
23
Other bases to use: n NaOCH3 in MeOH n KOH in EtOH n For primary alcohols
q KOC(CH3)3 in t-BuOH or DMSO E1 mechanism: n More likely for 3º RX
q And for RI > RCl q And weak or no bases q Solvent acts as base (EtOH) q Often show rearrangements (E2 not as much) q Favors mixtures of products q So to increase yield of single product, avoid E1
E2 mechanism proofs:
1. Rate = k[RX][base], 1. favored for strong bases 2. Any RX
24
Anti E2 : Stereoelectronic effects
Cis- isomer 500x faster rate than trans-
Reaction of Alkenes: Reaction with X2 Reaction with X2 in H2O
Cl
Cl
Br
Br
Cl2
Br2
OH
Br
Br2/H2O
25
Reduction of Alkenes: Hydrogenation
Mechanism of Catalytic Hydrogenation
Electrophilic Addition of HX to Alkenes: n General reaction mechanism: electrophilic addition n Attack of electrophile (such as HBr) on π bond of alkene n Produces carbocation and bromide ion n Carbocation is an electrophile, reacting with nucleophilic
bromide ion Writing Organic Reactions n No established convention – shorthand n Can be formal kinetic expression n Not necessarily balanced n Reactants can be before or on arrow n Solvent, temperature, details, on arrow
26
Example of Electrophilic Addition: n Addition of hydrogen bromide to 2-Methyl-propene n H-Br transfers proton to C=C n Forms carbocation intermediate
n More stable cation forms n Bromide adds to carbocation
Orientation of Electrophilic Addition: Markovnikov’s Rule: n In an unsymmetrical alkene, HX reagents can add in two
different ways, but one way may be preferred over the other
n If one orientation predominates, the reaction is regiospecific
n Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents)
This is Markovnikov’s rule Example of Markovnikov’s Rule n Addition of HCl to 2-methylpropene n Regiospecific – one product forms where two are possible n If both ends have similar substitution, then not
regiospecific
27
Energy of Carbocations and Markovnikov’s Rule n More stable carbocation forms faster n Tertiary cations and associated transition states are more
stable than primary cations
Mechanistic Source of Regiospecificity in Addition Reactions: n If addition involves a carbocation intermediate
n and there are two possible ways to add n the route producing the more alkyl substituted
cationic center is lower in energy n alkyl groups stabilize carbocation
28
Carbocation Structure and Stability: n Carbocations are planar and the tricoordinate carbon is
surrounded by only 6 electrons in sp2 orbitals n The fourth orbital on carbon is a vacant p-orbital n The stability of the carbocation (measured by energy
needed to form it from R-X) is increased by the presence of alkyl substituents
n Therefore stability of carbocations: 3º > 2º > 1º > +CH3 Transition State for Alkene Protonation: n Resembles carbocation intermediate n Close in energy and adjacent on pathway n Hammond Postulate says they should be similar in
structure
29
Mechanism of Electrophilic Addition: Rearrangements of Carbocations: n Carbocations undergo structural rearrangements following
set patterns n 1,2-H and 1,2-alkyl shifts occur n Goes to give more stable carbocation n Can go through less stable ions as intermediates
30
Oxidation of Alkenes: Epoxides: Epoxide Preparation
Cl
OOHO
O
H
mcpba
CH2Cl2
mcpba = peroxide
OH
BrO
Br2/H2O base
bromohydrin
31
Radical Reactions – HBr
• If reaction is done with HBr/peroxides • Get the non-Markovnikov product
Radical Reactions: Polymer Formation:
• Polymer – a very large molecule made of repeating units of smaller molecules (monomers)
• Biological Polymers • Starch • Cellulose • Protein • Nucleic Acid
Polymers: • Alkene polymerization • Initiator used generally is a radical
Mechanism:
• Initiation • Propagation • Termination
Br
HBr/peroxides
n
repeatingunit
32
Alkynes Reactions of Alkynes:
• Alkynes are rare in biological chemistry • Chemistry is similar to alkenes • Generally less reactive than alkenes • Reactions can be stopped at alkene stage using one
equivalent of the reagent Reactions with HX
• Regiochemistry is Markovnikov
Reactions with X2
• Initial addition gives trans intermediate • Product with excess reagent is tetra-halide
Reactions with H2
• Reduction using Pd or Pt does not stop • Alkene is more reactive than alkyne
33
Reactions with H2
• Lindler’s catalyst is poisoned • Not as reactive • Stops at cis-alkene
Reduction using dissolving metals
• Anhydrous ammonia (NH3) is a liquid below -33 ºC • Alkali metals dissolve in liquid ammonia • Provide a solution of e- in NH3 • Alkynes are reduced to trans alkenes with sodium or
lithium in liquid ammonia Hydration of Alkynes
• Hydration (Hg+2) of terminal alkynes provides methyl ketones
• Hydration (BH3) of terminal alkynes provides aldehydes
34
Alkyne Acidity: Acetylide Anion • Terminal alkynes are weak Brønsted acids • pKa is approximately 25 • alkenes and alkanes are much less acidic • Reaction of strong anhydrous bases with a terminal
acetylene produces an acetylide ion
Alkylation of Acetylide Anions
• Acetylide ions are nucleophiles • Acetylide ions are bases • React with a primary alkyl halides
35
Aromatic Compounds Discovery of Benzene
• Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1.
• Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C6H6.
Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic Kekulé Structure
• Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested.
• Failed to explain existence of only one isomer of 1,2-dichlorobenzene.
Resonance Structure Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring.
36
Unusual Reactions • Alkene + KMnO4 → diol (addition)
Benzene + KMnO4 → no reaction. • Alkene + Br2/CCl4 → dibromide (addition)
Benzene + Br2/CCl4 → no reaction. With FeCl3 catalyst, Br2 reacts with benzene to form bromobenzene + HBr (substitution!). Double bonds remain Unusual Stability Hydrogenation of just one double bond in benzene is endothermic
Annulenes
• All cyclic conjugated hydrocarbons were proposed to be aromatic.
• However, cyclobutadiene is so reactive that it dimerizes before it can be isolated.
• And cyclooctatetraene adds Br2 readily. Look at MO’s to explain aromaticity
37
MO Rules for Benzene
• Six overlapping p orbitals must form six molecular orbitals.
• Three will be bonding, three antibonding. • Lowest energy MO will have all bonding interactions, no
nodes. As energy of MO increases, the number of nodes increases. MO’s for Benzene
38
Energy Diagram for Benzene
• The six electrons fill three bonding pi orbitals. • All bonding orbitals are filled (“closed shell”), an
extremely stable arrangement.
Aromatic Requirements
• Structure must be cyclic with conjugated pi bonds.
• Each atom in the ring must have an unhybridized p orbital. • The p orbitals must overlap continuously around the ring.
(Usually planar structure.) Compound is more stable than its open-chain counterpart. Anti- and Nonaromatic
• Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but the energy of the compound is greater than its open-chain counterpart.
Nonaromatic compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar. Hückel’s Rule
• If the compound has a continuous ring of overlapping p orbitals and has 4N + 2 electrons, it is aromatic.
If the compound has a continuous ring of overlapping p orbitals and has 4N electrons, it is antiaromatic.
39
[N]Annulenes • [4]Annulene is antiaromatic (4N e-’s) • [8]Annulene would be antiaromatic, but it’s not planar, so
it’s nonaromatic. • [10]Annulene is aromatic except for the isomers that are
not planar. Larger 4N annulenes are not antiaromatic because they are flexible enough to become nonplanar. Cyclopentadienyl Ions
• The cation has an empty p orbital, 4 electrons, so antiaromatic.
The anion has a nonbonding pair of electrons in a p orbital, 6 e-
’s, aromatic.
Common Names of Benzene Derivatives
OH OCH3NH2CH3
phenol toluene aniline anisole
40
CH
CH2 C
O
CH3C
O
HC
O
OH
styrene acetophenone benzaldehyde benzoic acid