Chapter 1- CHM 261 organic chemistry

transcript

Organic Chemistry

Review of Molecular Structure

Chapter 1

Organic Chemistry

The study of the compounds of carbon Over 10 million compounds have been identified

• about 1000 new ones are identified each day! C is a small atom

• it forms single, double, and triple bonds• it is intermediate in electronegativity (2.5)• it forms strong bonds with C, H, O, N, and some metals

Schematic View of an Atom

• a small dense nucleus, diameter 10-14 - 10-15 m, which contains positively charged protons and most of the mass of the atom

• an extranuclear space, diameter 10-10 m, which contains negatively charged electrons

Electron Configuration of Atoms

Electrons are confined to regions of space called principle energy levels (shells)• each shell can hold 2n2 electrons (n = 1,2,3,4......)

Number of Electrons Shell

Can Hold

Relative Energiesof Electrons

in These Shells

3218 8 2

higher

Shells are divided into subshells called orbitals, which are designated by the letters s, p, d, f,........• s (one per shell) • p (set of three per shell 2 and higher) • d (set of five per shell 3 and higher) .....

Shell Orbitals Contained in That Shell

2s, 2px, 2py, 2pz

3s, 3px, 3py, 3pz, plus five 3d orbitals

Aufbau Principle: • orbitals fill in order of increasing energy from lowest

energy to highest energy Pauli Exclusion Principle:

• only two electrons can occupy an orbital and their spins must be paired

Hund’s Rule: • when orbitals of equal energy are available but there

are not enough electrons to fill all of them, one electron is added to each orbital before a second electron is added to any one of them

The pairing of electron spins

Table 1.3 The Ground-State Electron Configuration of Elements 1-18

Lewis Dot Structures

Gilbert N. Lewis Valence shell:

• the outermost occupied electron shell of an atom Valence electrons:

• electrons in the valence shell of an atom; these electrons are used to form chemical bonds and in chemical reactions

Lewis dot structure: • the symbol of an element represents the nucleus and

all inner shell electrons• dots represent valence electrons

Lewis Dot Structures

Lewis Dot Structures for Elements 1-18

SiAl P

1A 2A 3A 4A 5A 6A 7A 8A

::::::::

::::::.

Lewis Model of Bonding

Atoms bond together so that each atom acquires an electron configuration the same as that of the noble gas nearest it in atomic number• an atom that gains electrons becomes an anion• an atom that loses electrons becomes a cation• the attraction of anions and cations leads to the

formation of ionic solids• an atom may share electrons with one or more atoms

to complete its valence shell; a chemical bond formed by sharing electrons is called a covalent bond

• bonds may be partially ionic or partially covalent; these bonds are called polar covalent bonds

Covalent Bonds

The simplest covalent bond is that in H2

• the single electrons from each atom combine to form an electron pair

• the shared pair functions in two ways simultaneously; it is shared by the two atoms and fills the valence shell of each atom

The number of shared pairs• one shared pair forms a single bond• two shared pairs form a double bond• three shared pairs form a triple bond

H H H-H+ • H0 = -435 kJ (-104 kcal)/mol•

Lewis Structures

To write a Lewis structure• determine the number of valence electrons

• Atom by atom analysis• determine the arrangement of atoms

• Different arrangements are almost always possible - isomers• connect the atoms by single bonds• arrange the remaining electrons so that each atom has

a complete valence shell• Use common bonding patterns as your guide

• show a bonding pair of electrons as a single line• show a nonbonding pair of electrons as a pair of dots

Stable Bonding in Neutral Molecules

In neutral molecules• hydrogen has one bond• carbon has 4 bonds and no lone pairs• nitrogen has 3 bonds and 1 lone pair• oxygen has 2 bonds and 2 lone pairs• halogens have 1 bond and 3 lone pairs• 3rd row and transition metal bonding patterns are more

complicated but we don’t encounter a lot of them here

Multiple Bonding

Bonded atoms can share more than one pair of electrons to form multiple bonds

Molecular oxygen (O2) has a double bond between the oxygen atoms

+ givesO O O O

Lewis Structures -

H2O (8) NH3 (8) CH4 (8) HCl (8)

C2H4 (12) C2H2 (10) CH2O (12) H2CO3 (24)

H-O-H H-N-HH

H-C-HH

H-C C-HH

Ethylene

Hydrogen chlorideMethaneAmmoniaWater

Carbonic acidFormaldehydeAcetylene

Electronegativity

Electronegativity: • a very crude measure of an atom’s attraction for the

electrons it shares with another atom in a chemical bond

Pauling scale• generally increases left to right in a row• generally increases bottom to top in a column

Polar and Nonpolar Covalent Bonds

Although all covalent bonds involve sharing of electrons, they differ widely in the degree of sharing

We divide covalent bonds into• nonpolar covalent bonds• polar covalent bonds

Difference in ElectronegativityBetween Bonded Atoms Type of Bond

Less than 0.5

0.5 to 1.9Greater than 1.9

Nonpolar covalentPolar covalent

Ions form

Polar and Nonpolar Covalent Bonds

• an example of a polar covalent bond is that of H-Cl• the difference in electronegativity between Cl and H is

3.0 - 2.1 = 0.9• we show polarity by using the symbols d+ and d-, or

by using an arrow with the arrowhead pointing toward the negative end and a plus sign on the tail of the arrow at the positive end

H Cl+ -

Polar Covalent Bonds

Bond dipole moment (m): • a measure of the polarity of a covalent bond• the product of the charge on either atom of a polar

bond times the distance between the nuclei• average bond dipole moments of selected covalent

H-OH-NH-C

H-S C-I

C-FC-ClC-Br C-N

Bond Dipole (D) Bond

1.51.51.41.2

Bond Dipole (D)

1.30.3 0.7

0.20.7

Bond Dipole (D)Bond

Polar and Nonpolar Molecules

To determine if a molecule is polar, we need to determine • if the molecule has polar bonds• the arrangement of these bonds in space

Molecular dipole moment (): the vector sum of the individual bond dipole moments in a molecule• reported in debyes (D)

these molecules have polar bonds, but each has a zero dipole moment

Carbon dioxide = 0 D

Boron trifluoride = 0 D

ClClCl

Carbon tetrachloride = 0 D

these molecules have polar bonds and are polar molecules

Water = 1.85D

Ammonia = 1.47D

directionof dipolemoment

• formaldehyde has polar bonds and is a polar molecule

Formaldehyde = 2.33 D

directionof dipolemoment H H

Formation of Ions

A rough guideline: • ions will form if the difference in electronegativity

between interacting atoms is 1.9 or greater• example: sodium (EN 0.9) and fluorine (EN 4.0)• we use a single-headed (barbed) curved arrow to show

the transfer of one electron from Na to F

• in forming Na+F-, the single 3s electron from Na is transferred to the partially filled valence shell of F

Na + F Na+ F -

• •• •••

••

• •••

••

+ F-(1s22s22p6)Na+(1s22s22p6)F(1s22s22p5)+Na(1s22s22p63s1)

Formal Charge

Formal charge: the charge on an atom in a molecule or a polyatomic ion

To derive formal charge1. write a correct Lewis structure for the molecule or ion

2. assign each atom all its unshared (nonbonding) electrons and one-half its shared (bonding) electrons

3. compare this number with the number of valence electrons in the neutral, unbonded atom

Number of valence electrons in the neutral, unbonded atom

All unsharedelectrons

One half of all shared electrons

+Formalcharge

Formal Charge

Example: Draw Lewis structures, and show which atom in each bears the formal charge

NH2- HCO3

- CO32-

CH3NH3+ HCOO- CH3COO-

(b) (c)

(d) (e) (f)

Exceptions to the Octet Rule

Molecules containing atoms of Group 3A elements, particularly boron and aluminum

Aluminum chloride

6 electrons in the valence shells of boron

and aluminum

Boron trifluoride

Exceptions to the Octet Rule

Atoms of third-period elements have 3d orbitals and may expand their valence shells to contain more than 8 electrons• Sulfur, phosphorus may have up to 10

Resonance

For many molecules and ions, no single Lewis structure provides a truly accurate representation

Ethanoate ion(acetate ion)

Resonance

Linus Pauling - 1930s• many molecules and ions are best described by

writing two or more Lewis structures• individual Lewis structures are called contributing

structures• connect individual contributing structures by double-

headed (resonance) arrows• the molecule or ion is a hybrid of the various

contributing structures

Resonance

Examples: equivalent contributing structures

Acetate ion(equivalent contributing

structures)

Nitrite ion(equivalent contributing

structures)

Resonance

Curved arrow: a symbol used to show the redistribution of valence electrons

In using curved arrows, there are only two allowed types of electron redistribution:• from a bond to an adjacent atom• from an atom to an adjacent bond

Electron pushing is a survival skill in organic chemistry• learn it well!

Resonance

All contributing structures must1. have the same number of valence electrons

2. obey the rules of covalent bonding• no more than 2 electrons in the valence shell of H • no more than 8 electrons in the valence shell of a 2nd

period element• 3rd period elements, such as P and S, may have up to

12 electrons in their valence shells

3. differ only in distribution of valence electrons; the position of all nuclei must be the same

4. have the same number of paired and unpaired electrons

Resonance The carbonate ion, for example

• a hybrid of three equivalent contributing structures• the negative charge is distributed equally among the

three oxygens

Resonance

Preference 1: filled valence shells• structures in which all atoms have filled valence shells

contribute more than those with one or more unfilled valence shells

••

••••

Greater contribution; both carbon and oxygen have complete valence shells

Lesser contribution;carbon has only 6 electrons in its valence shell

+ +CCH3 OCH3 O

Resonance

Preference 2: maximum number of covalent bonds• structures with a greater number of covalent bonds

contribute more than those with fewer covalent bonds

••

••••

Greater contribution(8 covalent bonds)

Lesser contribution(7 covalent bonds)

+ +CCH3 OCH3 O

Resonance

Preference 3: least separation of unlike charge• structures with separation of unlike charges contribute

less than those with no charge separation

Lesser contribution(separation of unlike

charges)

CH3-C-CH3 CH3-C-CH3

Greater contribution(no separation of unlike charges)

Resonance

Preference 4: negative charge on the more electronegative atom • structures that carry a negative charge on the more

electronegative atom contribute more than those with the negative charge on the less electronegative atom

CH3CH3H3CCH3H3CC

(b)Greater

contribution

(c)Should notbe drawn

(a)Lesser

contribution

(1) (2)

Structural Formulas

Lewis structures indicate atom connectivity• Do not show 3-D structure, but it is there

Drawing Lewis structures of large molecules can get cumbersome• Use shortcuts to simplify things• Rely on normal bonding properties of atoms

Types include• Condensed structural formulas• Line-angle formulas

All subsets of molecular formula

Condensed Structural formulas

Based on typical bonding patterns• Carbon makes 4 bonds in stable molecules• Oxygen makes 2 bonds and has 2 lone pairs• Nitrogen makes 3 bonds and has 1 lone pair

Show molecule by “groups” centered on carbon

CH3CH3

CH3CH2CH3

(CH3)2CH2

CH3CHCH3

CH3CH(CH3)CH3

(CH3)3CH

Which is the correct Lewis structure?

(CH3)2CH(CH2)2CH(CH3)2

Can add heteroatoms (N, O, halogens) and multiple bonds

CH3CH2OH

CH3CH2NHCH3

CH3CHClCH3

CH3CH=CHCH3

Line-Angle structures

Lines to represent bonds • “skeletal structures”

Carbons are not drawn, but occur at• Intersections• Vertices (ends of lines)

Hydrogens are not usually shown• assumed to be as many as needed to fulfill carbon

valency

CH3(CH2)4CH3

A. 0 B. 1 C. 2 D. 3 E. 4

Heteroatoms are shown explicitly WITH Hydrogens

How many hydrogens are present at marked position?

Combined structures are allowed BUT…

…if you show C atoms, you MUST show the 4 bonds to it (including H)

H3C OH

CH(CH3)2

NOT OK

C C C C

(C14H30)

Trivalent carbon

Find the acceptable structure

Trivalent carbon can be possible for ions and radicals

one missing electron around carbon(3 net electrons)

one extra electron around carbon(5 net electrons)

One for each bond (3) – no more (electron deficient)

3 from the bonds, must have a non-bonded lone pair (note that it has an octet even!)

Pentavalent carbon (5 bonds to C) is NEVER acceptable in this class

Acids and Bases

Arrhenius Acids and Bases

In 1884, Svante Arrhenius proposed these definitions • acid: a substance that produces H3O+ ions aqueous

solution• base: a substance that produces OH- ions in aqueous

solution• this definition of an acid is a slight modification of the

original Arrhenius definition, which was that an acid produces H+ in aqueous solution

• today we know that H+ reacts immediately with a water molecule to give a hydronium ion

H+(aq) + H2O(l) H3O+(aq)Hydronium ion

Brønsted-Lowry Definitions

Acid: a proton donor Base: a proton acceptor

Proton donor

Protonacceptor

O HH + +O H H

Protonacceptor

Protondonor

H + +N H H

Conjugate Acids & Bases

• conjugate base: the species formed from an acid when it donates a proton to a base

• conjugate acid: the species formed from a base when it accepts a proton from an acid

• acid-base reaction: a proton-transfer reaction• conjugate acid-base pair: any pair of molecules or ions

that can be interconverted by transfer of a proton

HCl(aq) H2O(l) Cl-(aq) H3O+(aq)+ +WaterHydrogen

chlorideHydronium

ionChloride

ion(base)(acid) (conjugate

acid of H2O)(conjugate

base of HCl)

conjugate acid-base pair

• Brønsted-Lowry definitions do not require water as a reactant

• consider the following reaction between acetic acid and ammonia

NH4+CH3COOH CH3COO-

NH3+ +

Acetic acid Ammonia

(acid)

Acetate ion

Ammoniumion

(base) (conjugate baseacetic acid)

(conjugate acidof ammonia)

• we can use curved arrows to show the flow of electrons in an acid-base reaction

CH3-C-OO

H N HH

H CH3-C-O -

OH-N-H

Acetic acid(proton donor)

Acetate ion

:: :: :

Ammonia(proton acceptor)

Ammoniumion

Many organic molecules have two or more sites that can act as proton acceptors in these molecules, the favored site of protonation is

the one in which the charge is more delocalized question: which oxygen of a acetic acid is protonated?

CH3-C-O-H

O+ H2SO4 CH3-C-O-H

CH3-C-O-HH

+ HSO4-or

A(protonation

on the carbonyl oxygen)

B(protonation

on thehydroxyl oxygen)

acetic acid

for protonation on the carbonyl oxygen, we can write three contributing structures

two place the positive charge on oxygen, one places it on carbon

A-1 and A-3 make the greater contribution because all atoms have complete octets

the positive charge is delocalized over three atoms with the greater share on the two equivalent oxygens

CH3-C-O-H

++CH3-C-O-H

CH3-C=O-H

A-1(C and O have

complete octets)

A-2(C has incomplete

octet)

A-3(C and O have

complete octets)

for protonation on the hydroxyl oxygen, we can write two contributing structures

B-2 makes only a minor contribution because of charge separation and adjacent positive charges

therefore, we conclude that protonation of a carboxylic acid occurs preferentially on the carbonyl oxygen

CH3-C-O-H

B-2(charge separation and

adjacent positive charges)

Does proton transfer to acetamide occur preferentially on the oxygen or the nitrogen?

CH3-C-N-H

O+ HCl

CH3-C-N-H

+ Cl -or

A(protonation

on theamide oxygen)

B(protonation

on theamide nitrogen)

Acetamide(an amide)

Acids & Base Strengths

The strength of an acid is expressed by an equilibrium constant the acid dissociation of acetic acid is given by the

following equation

+CH3CO-

OH2O+CH3COH

Acetic acid Water Acetateion

Hydroniumion

Weak Acids and Bases

We can write an equilibrium expression for the dissociation of any uncharged acid, HA, as:

water is a solvent and its concentration is a constant equal to approximately 55.5 mol/L

we can combine these constants to give a new constant, Ka, called an acid dissociation constant

HA + H2O

[H3O+][A

[HA][H2O]Keq

A- + H3O

Keq[H2O]Ka[H3O+][A-]

[HA]= =

1-62HI I-

HBr Br-

HCl Cl-

H2SO4 HSO4-

H2OH3O+

H3PO4 H2PO4-

C6H5COOH C6H5COO-

CH3COOH CH3COO-

H2CO3 HCO3-

H2S HS-

NH3NH4+

C6H5OH C6H5O-

CH3NH2CH3NH3+

H2O HO-

CH3CH2OH CH3CH2O-

HC CH HC C-H2 H-NH3 NH2

-CH2=CH2 CH2=CH

-CH3CH3 CH3CH2

-Acid Formula pKa Conjugate BaseEthane

Ammonia

EthanolWater

Bicarbonate ionPhenol

Ammonium ion

Carbonic acidAcetic acid

Benzoic acidPhosphoric acid

Sulfuric acidHydrogen chlorideHydrogen bromideHydrogen iodide

15.715.9

4.766.36

-5.2-7

2.1-1.74Hydronium ion

Strongerconjugate

Weakerconjugate

Weaker acid

Stronger acid

Methylammonium ion 10.64

Hydrogen sulfide 7.04

AcetyleneHydrogen

Ethylene 44

Acid-Base Equilibria

Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base

CH3COOH NH3 CH3COO-

NH4++ +

Ammonia(stronger base)

Acetate ion(weaker base)

Acetic acidpKa 4.76

(stronger acid)

Ammonium ionpKa 9.24

(weaker acid)

pKeq = 4.76 - 9.24 = -4.48

Keq = 3.0 x 104

Acid-Base Equilibria

Consider the reaction between acetic acid and sodium bicarbonate we can write the equilibrium as a net ionic equation we omit Na+ because it does not undergo any

chemical change in the reaction

equilibrium lies to the right carbonic acid forms, which then decomposes to

carbon dioxide and water

CH3COH

- CH3CO-

Bicarbonate ion Acetate ionAcetic acidpKa 4.76

(stronger acid)

Carbonic acidpKa 6.36

(weaker acid)

Lewis Acids and Bases

Lewis acid: any molecule of ion that can form a new covalent bond by accepting a pair of electrons

Lewis base: any molecule of ion that can form a new covalent bond by donating a pair of electrons

Lewis base

Lewis acid

+ BA new covalent bondformed in this Lewisacid-base reaction

Lewis Acids and Bases

• examples

Br -CH3-C C-CH3

CH3-C C-CH3

Bromideion

sec-Butyl cation(a carbocation)

2-Bromobutane

Diethyl ether(a Lewis base)

Boron trifluoride (a Lewis acid)

CH3CH2

A BF3-ether complex

Lewis Acids include Bronsted Acids• A proton (H+) is a Lewis acid

• Proton donors therefore “accept a pair of electrons” to form a new covalent bond – between the base and proton

• A covalent bond is also broken but that doesn’t matter• The definition of Lewis acid doesn’t say anything about not

breaking bonds • All that matters is that it can ACCEPT a pair of electrons

• Hydroxide (OH) is clearly a Lewis base• Other things are bases in Lewis approach (ammonia, halide

• Most neutral molecules are acids and bases• However, they can be very different in acid/base strength NH2

+ H3O+ :NH3 + H2O NH4+ + OH

pKa(NH3) = 38 pKa(NH4+) = 9

Chapter 1- CHM 261 organic chemistry

Documents