Lecture 8 - Alkenes & Alkynes

General

Organic Chemistry Two credits Second Semester 2009

King Saud bin Abdulaziz University for Health Science

Reference Book: Organic Chemistry: A Brief Course, by Robert C. Atkins and Francis A. Carey Third Edition

Instructor: Rabih O. Al-Kaysi, PhD.

Lecture 8

Chapter 4

Alkenes & Alkynes

Alkynes

Sources of Alkynes

Acetylene Acetylene Industrial preparation of acetylene is by dehydrogenation of ethylene

CH3CH3 H2C

CH2

800°C 1150°C

H2C

CH2 + H2

HC

CH + H2

cost of energy makes acetylene a more expensive industrial chemical than ethylene

Nomenclature

Nomenclature Nomenclature Acetylene and ethyne are both acceptable IUPAC names for HC CH Higher alkynes are named in much the same way as alkenes except using an -yne suffix instead of -ene. HC

CCH3

Propyne (CH3)3CC

HC

CCH2CH3

1-Butyne CCH3

4,4-Dimethyl-2-pentyne

Physical Properties of Alkynes

The physical properties of alkynes are similar to those of alkanes and alkenes.

Structure and Bonding in Alkynes: sp Hybridization

Structure Structure linear geometry for acetylene

120 pm H C C H 106 pm 106 pm 121 pm CH3 C C H 146 pm 106 pm

Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperature for a reasonable length of time. Cyclooctyne polymerizes on standing.

Just for general knowledge, will not be tested on

Bonding Bondingininacetylene acetyleneisisbased basedon onsp sp-hybridization -hybridization for foreach eachcarbon carbon Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p

2p

2sp 2s

Bonding Bondingininacetylene acetyleneisisbased basedon onsp sp-hybridization -hybridization for foreach eachcarbon carbon Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p Each carbon has two 2sp half-filled sp orbitals available to form σ bonds.

σσ Bonds BondsininAcetylene Acetylene

Each carbon is connected to a hydrogen by a σ bond. The two carbons are connected to each other by a σ bond and two π bonds.

ππ Bonds BondsininAcetylene Acetylene

One of the two π bonds in acetylene is shown here. The second π bond is at right angles to the first.

ππ Bonds BondsininAcetylene Acetylene

This is the second of the two π bonds in acetylene.

Comparison of ethane, ethylene, and acetylene Ethane

Ethylene

Acetylene

C—C distance

153 pm

134 pm

120 pm

C—H distance

111 pm

110 pm

106 pm

H—C—C angles

111.0°

121.4°

180°

hybridization of C

sp33

sp22

sp

25%

33%

50%

% s character

Acidity of Acetylene and Terminal Alkynes

H

C

C

Acetylene Acetyleneisisaaweak weakacid, acid,but butnot notnearly nearly as asweak weakas asalkanes alkanesor oralkenes. alkenes.

Compound

HC

CH

pK a

HF H2 O

3.2 16

NH3

36

H245 C CH2 CH4

26

60

Just for general knowledge, will not be tested on

Solution: Solution: Use Useaastronger strongerbase. base. Sodium Sodiumamide amide isisaastronger strongerbase basethan thansodium sodiumhydroxide. hydroxide.

NaNH2 + HC .. – H2N : +

H

CH

C

CH

stronger acid pKa = 26

NaC

CH + NH3

.. H2N

– H + :C

weaker acid pKa = 36

Ammonia is a weaker acid than acetylene. TheJust position of equilibrium liesnot to be the right. for general knowledge, will tested on

CH

Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes

Preparation Preparationof ofAlkynes Alkynes There are two main methods for the preparation of alkynes: Carbon-carbon bond formation alkylation of acetylene and terminal alkynes Functional-group transformations elimination

Alkylation Alkylationof ofacetylene acetyleneand andterminal terminalalkynes alkynes H—C

C—H

R—C

C—H

R—C

C—R

Alkylation Alkylationof ofacetylene acetyleneand andterminal terminalalkynes alkynes

H—C

– C: +

R

X

H—C

C—R + : X–

• The alkylating agent is an alkyl halide, and the reaction is nucleophilic substitution. • The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) alkyne.

Example: Example: Alkylation Alkylationof ofacetylene acetylene

HC

CH

NaNH2

HC

NH3

CNa CH3CH2CH2CH2Br

HC

C

CH2CH2CH2CH3 (70-77%)

Example: Example: Alkylation Alkylationof ofaaterminal terminalalkyne alkyne (CH3)2CHCH2C

CH

NaNH2, NH3

(CH3)2CHCH2C

CNa

CH3Br (CH3)2CHCH2C (81%)

C—CH3

Preparation of Alkynes by Elimination Reactions

Preparation Preparationof ofAlkynes Alkynesby by "Double" "Double"Dehydrohalogenation Dehydrohalogenation

H

H

C

C

X

X

Vicinal dihalide The most frequent applications are in preparation of terminal alkynes.

Vicinal Vicinaldihalide dihalide→ →Alkyne Alkyne CH3(CH2)7CH—CH2Br Br 1. 3NaNH2, NH3 2. H2O CH3(CH2)7C (54%)

CH

Hydrogenation of Alkynes

Hydrogenation Hydrogenationof ofAlkynes Alkynes

RC

CR'

+ 2H2

cat

RCH2CH2R'

catalyst = Pt, Pd, Ni, or Rh

alkene is an intermediate

Example Example CH3(CH2)3C

C(CH2)3CH3 + H2 Lindlar Pd

CH3(CH2)3

(CH2)3CH3 C

C H

H (87%)

Addition of Hydrogen Halides to Alkynes

Follows FollowsMarkovnikov's Markovnikov'sRule Rule

CH3(CH2)3C

CH

HBr

CH3(CH2)3C

CH2

Br (60%)

Alkynes are slightly less reactive than alkenes

Termolecular Termoleculartransition transitionstate state

H

RC

H

.. Br : ..

CH

.. Br : ..

Reaction Reactionwith withtwo twomoles molesof ofaahydrogen hydrogen halide halideyields yieldsaageminal geminaldihalide dihalide CH3CH2C

CCH2CH3 2 HF

CH3CH2

H

F

C

C

H

F

(76%)

CH2CH3

Hydration of Alkynes

Hydration Hydrationof ofAlkynes Alkynes

expected reaction: RC

CR'

+ H2O

H+

RCH

CR' OH

enol

Addition of Halogens to Alkynes

Example Example

Cl HC

CCH3 + 2 Cl2

Cl2CH

C

CH3

Cl (63%)

Addition Additionisisanti anti

CH3CH2C

CCH2CH3

Br2

CH3CH2

Br C

C CH2CH3

Br (90%)