Synthesis of Alkenes and Alkynes

Question 1

Assigning “E” and “Z” Diastereomers

Answer 1

1. Assign priority of substituents
2. Chose the highest priority on each end
3. Compare two highest priority substituents
   - Same side Z
   - Opposite side E

Question 2

When are alkenes most stable?

Answer 2

Most substituted alkenes are more stable

tetrasubstituted > trisubstituted > disubstituted ( same side > trans > cis) > monosubstituted > unsubstituted

Question 3

Dehydrohalogenations (E2)

Answer 3

• elimination of HX

Question 4

Zaitsev’s Rule

Answer 4

E2 with UNHINDERED BASE gives the MOST stable alkene

Question 5

Hofmann’s Rule

Answer 5

E2 with HINDERED BASE gives the LEAST stable alkene

Question 6

Dehydrations (E1, mostly)

Answer 6

elmination of HOH (water!)

Question 7

Dehydration of Alcohols

Answer 7

• primary (slow E2, since weak base): HARDEST TO HYDRATE
• Secondary Alcohol (E1)
• Tertiary Alcohol (E1) - EASIEST TO DEHYDRATE!

Question 8

S character for ethane

Answer 8

25%

Question 9

S character for ethene

Answer 9

33%

• electrons are close to C nucleus
• vinylic C-H bond is shorter, stronger and difficult to break

Question 10

S character for ethyne

Answer 10

50%

sp-C pulls C-H electrons towards itself, creating a partial positive charge on the H (becomes acidic!)

Question 11

Trends in acidity

Answer 11

• weak bases not strong enough to deprotonate alkyne (-OH, -OR)
• strong bases enought to deprotonate alkyne (-H, -NR2, -CR3)

Question 12

Key Features of Dihalide E2 Reactions

Answer 12

1. “Anti-coplanar” -H must have 180deg dihedral angle LG

2. Two pi bonds (alkyne) forms from between the four new p-orbitals

Question 13

Synthesis of Terminal Alkynes

Answer 13

• Use >2eq Strong Base (NH2-)
  1. 1eq NH2- halide –> alkene E2 slow
  2. 1eq NH2- alkene –> alkyne E2 slow
  3. 1 eq NH2- alkyne R-C(triple)C-H –> R-C(triple)C- fast acid/base
  —————————
  Work up: R-C(triple)C- —>(H+) R-C(triple)C-H which is a separate reaction