Chemistry, Third edition

17 · ORGANIC CHEMISTRY: HYDROCARBONS

Naming alkenes

The following additional rules apply when naming alkenes:

1.Choose the longest chain of carbon atoms that contains the double bond.

2.Number the carbon atoms in the chain from the end nearest the double bond.

3.Pick the carbon atom with the lowest number to describe the position of the

double bond.

316

Table 17.2The first three alkenes

Name Molecular formula Structural formula

Ethene C 2 H 4 HH

||

C==C

||

HH

Propene C 3 H 6 HHH

|||

H—C—C==C

||

HH

But-1-ene C 4 H 8 HHHH

||||

H—C—C—C==C

|| |

HH H

Ethene molecule

(i)Draw a Lewis structure

to show the

arrangement of the

outer electrons of the

atoms in an ethene

molecule.

(ii)Make a model of the

ethene molecule. Is

there free rotation about

the carbon–carbon

double bond? How does

this compare with

ethane?

Exercise 17I

Example 17.3

43 2 1

CH 3 CH 2 CH=CH 2 is called but-1-ene notbut-2-ene

Example 17.4

CH 3

|

CH 3 CCH=CH 2 is called 3,3-dimethylbut-1-ene

|

CH 3

Naming alkenes 1

Name the following:

(i) CH 3 CH 2 CH 2 CH 2 CH=CH 2 (ii)CH 3 CH=CHCH 3

CH 3

|

(iii)CH 3 CCH=CHCH 3 (iv)CH 2 =CHCH 2 Br

|

CH 3

Exercise 17J

Naming alkenes 2

Write structural formulae for:

(i) hex-3-ene

(ii) 2-methylpropene

(iii)2,3-dimethylhex-2-ene

(iv)3-chloro-2-methylpropene

(v) 3-bromohex-1-ene.

Exercise 17K