Unsaturated HydrocarbonsAlkynes

1

1435-14362014-2015

Learning Objectives

Chapter three discusses the following topics which have to be understood and memorized :

The structure, hybridization and Bonding in alkynes

Common and IUPAC naming of alkynes

Physical properties of alkynes

Preparation of alkynes

Reactions of alkynes: addition reactions and acidity

Alkynes

Alkynes: Molecular And Structural Formulae The alkynes comprise a series of carbon- and hydrogen- based compounds that

contain at least one triple bond. This group of compounds is a homologous series with the general molecular formula of Cn H2n—2

The alkyne triple bond is composed of one σ and two 2 covalent bonds, the triple bond can be terminal or internal.

The simplest alkyne, ethyne (also known as acetylene), has two carbon atoms and the molecular formula of C2H2. The structural formula for ethyne is:

CH CH3

Alkynes

sp Hybridization Of Alkynes

This involves the mixing of one s- and one p-orbital forming two sp-hybrid orbitals of equivalent energy. The two sp-hybrid orbitals are oriented in a linear arrangement and bond angle is 180° to minimize the repulsion between them.

The remaining two p orbitals (py and Pz) are unaltered

4

Alkynes

CH CH

Molecular formula of ethyne is C2H2.

In ethyne, each carbon atom is sp-hybridized. In this way, four sp-orbital are generated. One sp- orbital of each carbon atom by overlapping forms a sigma

bond between carbon atoms. Remaining one sp-orbital of each carbon atom overlap with 1s-

orbital of a hydrogen atom to produce two sigma bonds. Py-orbital and Pz-orbitals of each carbon by parallel overlapping

form two pi-bonds between the two carbon atoms. Geometry (shape) of ethyne molecule is linear in which bond

angles are 180o and C=C bond length is 1.20 Å5

Orbital Overlap IN Ethyne

Alkynes

2s22p2 2s12p3 2 x sp 2p

sp HYBRIDISATION OF ORBITALS in ALKYNES

The electronic configuration of a carbon atom is 1s22s22p2

promotion hybridization

Alkynes

7

Alkynes

Summary

sp hybridization occurs when a C has 2 sigma bonds only sp hybridized orbital has 50% s and 50% p character The 2 sp hybrids point in opposite directions at 180o to each other Each sp hybrid is involved in a(σ)sigma bond The remaining p orbitals form the 2pi bonds The triple bond is one (σ)bond and two pi (∏) bonds.

8

Alkynes

Find the longest chain containing both atoms of the triple bond; this gives the root name.

Add the ending –yne to the root name.Number the chain, starting at the end closest to the triple bond. Give branches or other substituents names and numbers to locate their positions.Indicate the number of identical groups by prefixes di, tri, tetra, etc.Place the position numbers and names of the substituent groups in alphabetical

order, before the root name. In alphabetizing ignore prefixes like tert., di, tri, etc. but include iso and cyclo.

Double and triple bonds are considered to have equal priority: thus in a molecule with both a double and triple bond, whichever is close to the end of the chain determines the direction of numbering.

In case where double and triple bonds would have the same position number, the double bond takes the lower number.

In writing the final name ‘’ene’’ comes before ‘’yne’’ regardless which takes the lower number (i.e. alphabetical order).

IUPAC Nomenclature of Alkynes

9

Alkynes

ExamplesIUPAC Names Of Alkynes

C CHHC

CH3

CH

F

F

4,4-Difluoro-3-methyl-1-butyne

C CH-CH2-CH3

H

CHC

Hex-3-en-1-yne(triple bond closer to the end of chain)Note: An''e'' is dropped from ''ene''

due to it is followed by a vowel

HC C CH2 CH CH2

Pent-1-en-4-ynedouble and triple bonds have have the same position number

thus ene take lower number

1

2345

6-Ethyl-4-nonyne

C C

Br

H3C

5-Bromo-2-pentyneNot 1-Bromo-3-pentyne

1 2 34

6

1 2

3 4 5

7

8

9

1

5

1 2 3 4 5 6

234

Alkynes

Common Nomenclature Of Alkynes The simplest alkyne its common name is acetylene Therefore the common names of alkynes are derived from acetylene

( e.g. Methyl acetylene)

Examples:

C CH3C

CH3 C CH CH3 CH

CH3

CH2 C C CH

CH3

CH3

Common : IsobutyisopropylacetyleneCommon : Methyl acetylene

Common: Isopropylmethylacetylene

HC CH

Alkynes

Exercise

1) Give the IUPAC and common names of the following compounds

12

C CH

C CH2

CH2

C HC C C(CH3)3

a) b)

c) d) -

Alkynes

Physical Properties

Nonpolar, insoluble in water. Soluble in most organic solvents.The boiling points and melting points of alkynes increase with molecular weight. Branching reduces the boiling point of alkynes Terminal alkynes, R-CC-H, are more acidic than other

hydrocarbons.

13

Alkynes

C CH

C CH2

CH2

C HC C C(CH3)3

a) b)

c) d) -

Preparation of alkynes

1. By dehydrohalogenation of geminal or vicinal dihaloalkanes

(-2HX):

14

Alkynes

C C

XX

Strong base

- 2 HXC C

Br

Br

1,2-dihaloalkane

Excess NaNH2

- 2HBr

Br

Br

Br

KOH/ alcohol

heat

NaNH2

heat

15

2. From reaction of sodium Acetylide with Primary Alkyl Halides (Alkylationof acetylide with primary alkyl halid)

Alkynes

C CHR + Na liq NH3 C C

- Na

+R + H21/2

C C- Na

+R + R' X C CR R' + NaX

Sodium acetylide

For Terminal CC (use only Acetylene H-CC-H)

For Non Terminal CC (use Monosub. R-CC-H)

Na

NaNH2/ NH3 CH3Br

- NaBr

16

Alkynes

Reactions of alkynes

1. Addition of hydrogen ( Hydrogenation)

17

Alkynes can be reduced to trans-alkenes using Na or Li in liquid NH3

Alkynes can be partially reduced to cis-alkenes with H2 in the presence of poisoned catalysts.

HH

+ H2C C

Pd(BASO4) or (NiB)

Cis- alkene

H

H

+ H2C C

Na or Li / liq. NH3

Trans -alkene

Electrophilic Addition Reaction

Alkynes

H2/ Pd

or (Z)

or (E)

Pd(BaSO4): Lindlar’s Catalyst

2. Addition of halogen (Halogenation)

18

HCCH + X2 HXC=CXH + X2 H XX C–CXXH

If we used one mole ……(alkene) two mole …….alkane

Alkynes

C C C C

X

X

+ X2 + X2 C C

X X

XX

Tetrahalide(X= Br or Cl)

Trans

+ Br2

Br

Br

+ Br2

Br

Br

Br

Br

19

3 .Addition of hydrogen halide

HC CR + HX HHC=CX R + HX HHHC – CXXR

+ HBr

Br

+ HBr

Br

Br

Alkynes

C C C C

H X

+ HX + C C

H X

XH

A gem dihalide(X= Br , Cl, I)

HX

A haloalkene

20

4 .Addition of water: Hydration

Alkynes

HC

Propyne (an unstable enol)

C CH3

/ H2SO4, HgSO4 H-OH

Acetone(a stable carbonyl compound)

H

C

H

C

CH3

O HCH3C

O

CH3

H C

Acetylene

Specific example:

C H/ H2SO4, HgSO4 H-OH H

H

Acetaldehyde(a stable carbonyl compound)

HH

C

H

Vinyl alcohol(an unstable enol)

C

H

O HC

O

CH

HO-HH2SO4 / HgSO4C C +

O

H

an enol unstable

O

H

carbonyl more stable

H

Hketo-enol tautomerism

21

1. An alkyne’s name ends with(a) –ane (b) -ene(c) –yne (d) diene2. An alkyne function has …….. pi bond(s).(a) one (b) two(c) three (d) four3. Alkynes react with HCl by a mechanism called(a) elimination (b) Markovnikov addition(c) (d) substitution4. Alkynes react with water in the presence of a catalyst to give(a) a dialcohol (diol) (b) an alkane(c) an enol (d) a dibromide5. The conversion of alkynes to alkanes is an example of(a) oxidation (b) reduction(c) chlorination (d) dehydration

Alkynes

Thank You for your kind attention !

Questions?

Comments

22

Alkynes