Alkenes

Question 1

Issues with using the dehydration of alcohols to give alkenes

Answer 1

Gives rise to a mixture of products that are difficult to separate and rearrangements of the carbocation intermediate can occur

Question 2

Wittig reaction outline

Answer 2

Formation of an alkene from an aldehyde and an ylid

Question 3

Benefits of the Wittig reaction and con

Answer 3

chemo selective and regiospecific. The triphenyl phosphine oxide by product can contaminate the alkene and be difficult to remove

Question 4

Types of ylids

Answer 4

Reactive, moderated and stabilised ylids.

Question 5

Reactive ylids in the wittig reaction

Answer 5

R group is an alkyl. It requires a strong base such as NaH for its formation and the Z alkene dominates

Question 6

Moderated ylides in Wittig reactions

Answer 6

R group is Ph or vinyl. There is some stabilised charge but still requires a base such as NaH. Forms a mixture of E and Z alkenes

Question 7

Stabilised ylides in Wittig reactions

Answer 7

R is an electron withdrawing group such as esters etc. Requires a weak base such as Na2CO3 to be formed, gives mostly E alkenes and reacts ONLY with aldehydes

Question 8

Reduction of alkynes conditions

Answer 8

To give the Z product, H2 w Lindlar’s catalyst.
To give the E product, Na, liquid NH3 and H2O

Question 9

What is Lindlar’s catalyst

Answer 9

Its a poisoned Pd catalyst to reduce its catalytic activity. Pd-CaCO3, Pb(OAc)2

Question 10

Outline the Diels-Adler reaction

Answer 10

Occurs between conjugated dienes and an alkene. It is a 4pi + 2pi cycloaddition process. Needs a lewis acid

Question 11

Use of an alkyne during Diels-Adler reactions

Answer 11

Possible in place of the alkene but the diene must have electron withdrawing groups on it

Question 12

Making an alkyne nucleophile

Answer 12

BuLi removes the terminal proton to give the nucleophile

Question 13

Draw an ylid

Answer 13

notes