Alkenes, Alkynes Pt 1

Alkenes, Alkynes & EA Reaction Roldan M. de Guia Department of Biochemistry Faculty of Pharmacy University of Santo Tomas

CnAlkenes H2n

Alkynes

Alkenes

Cycloalkenes

Alkynes

Alkenes

Physical Properties • Alkenes & alkynes are nonpolar • Physical properties are similar to those of alkanes with same C skeletons • Those that are liquid at RT have densities <1.0g/mL

Physical Property: Boiling point



Alkenes have diffuse e- cloud; alkanes have stronger interaction; & alkyne’s π e- cloud is tighter than alkenes

ISOMERISM

STRUCTURE OF ALKENES

Alkenes: cis & trans isomerism cis

trans

H

H

R

H

R

R

H

R

All substituents are All substituents are on different sides on one side of π bond of π bond

Alkenes: cis & trans isomerism

Alkenes: cis & trans isomerism

Alkenes: cis & trans isomerism

Cis/trans Isomerism in Cycloalkenes • For cycloalkenes with 7 or fewer C atoms, the cis configuration is preferred

Cis/trans Isomerism in Di-, Tri-, & Polyenes • Total # of cis-trans isomers possible = 2n n = # of double bond Double bond C2 – C 3 C4 – C 5

2,4-heptadiene

trans

trans

trans

cis

cis

trans

cis

cis

What is the correct systematic name for the following? Br

• 1-bromo-2-hexene • 6-bromo-4-hexene • cis-1-bromo-2hexene • trans-1-bromo-2hexene

Alkenes: E/Z System For more than two substituents the cis/trans, the E/Z system is used.

Alkenes: E/Z System I

H

H3C H

H3C

F

(E)-3-fluoro-2-iodo-2-propene

H

CH2 OH CH2 CH3 path goes to O, not H

C O H H

comparison stops here

C C H

path goes to C, not H

Alkenes: E/Z System C H

H H3C

CH CH2 CH2 CH3

CH CH2

C C C H H

H H CH2 CH3

C C H H H