Chapter 3

Unsaturation

Recall:pentane – C5H12

We refer to pentane as a saturated compound because the carbon atoms have the maximum number of possible substituents

cyclopentane – C5H10

We refer to cyclopentane as an unsaturated compound because the carbon atoms do not have the maximum number of possible substituents

Can you think of any other ways to make an unsaturated molecule?

Double & Triple Bonds

C5H10

C5H10

C5H8

C5H8

C5H12

0° of unsaturation

Degree of unsaturation = (# of double bonds) + (2 × # of triple bonds) + (# of rings)

1° of unsaturation 2° of unsaturation

Multiple sites of unsaturation:

Double & Triple Bonds

C5H10

C5H10

C5H8

C5H8

C5H12

0° of unsaturation

Degree of unsaturation = (# of double bonds) + (2 × # of triple bonds) + (# of rings)

1° of unsaturation 2° of unsaturation

CnH2×n+2 CnHn CnH2×n-2

Double & Triple Bonds

C5H10

C5H10

C5H8

C5H8

C5H12

0° of unsaturation 1° of unsaturation 2° of unsaturation

Degree of unsaturation = [2 + (2 x #Cs) + #Ns - #Hs - #Halogens]/2

CnH2×n+2 CnHn CnH2×n-2

C6H10BrCl

C8H10NOBr

1°

4°

Double & Triple Bonds

Alkenes Alkynes

C5H10

C5H10

C5H8

C5H8

Alkanes

C5H12

pentane(n-pentane)

1-pentene

How do we name these guys?

2-pentene

1-pentyne

2-pentyne

Rules for Naming Alkenes and Alkynes

1) Find the longest carbon chain containing the multiple bond

2) Start numbering from the end closest to the multiple bond

3) Finish as if you were naming an alkane only use the appropriate suffix(e.g., -ene for a double bond, -yne for a triple bond)

Let’s try a practice problem

Name That Organic Compound

pentene1-3,4-dimethyl-

Name That Organic Compound

1,3-pentadiene

Nomenclature of Alkenes

Common Terms You Should Know:

vinylic carbons

allylic carbons

vinyl group

allyl group

2-pentene 12 3 4 5 6 7 8 9

10

5-allyl-4-vinyl-decane

Alkenes are also called olefins.

Alkene Factoids

Recall:CH3

H HCH3

H HH

H CH3CH3

H HH

H3C HCH3

H Hrotate rotate

butane(n-butane)

Now let’s look at 2-butene

H3C

H CH3

H

Alkene Factoids

The barrier to rotation is very large. This leads to the possibility of cis and trans isomers, stable, separable compounds with different properties.

E,Z Nomenclature

(The higher the molcular weight, the higher the priority)

Sometimes cis and trans are ambiguous; in such cases we resort to the more formal E,Z nomenclature.

cis or trans?

•Z if highest priority substituents on same side of double bond

•E if highest priority substituents on opposite side of double bond

A way to remember: Ecross, Zame Zide

•Look at each carbon atom and assign each group a priority

Practice

I

(Z)-2-iodo-2-pentene

Br

Br

1,1-dibromo-1-propene

Practice