Topic 2-2.5: Retrosynthesis and Alkynes

Question 1

what is functional group interconversion

Answer 1

conversion of one functional group to another

Question 2

how is alkyne prepared

Answer 2

via double dehydrohalogenation (elimination) of 1,2-dihaloalkene (vicinal dihalide)

1. vicinal dihalide prepared via addition of Br2 or Cl2 to alkene
2. vicinal dihalide treated with strong base (KOH/NaOH) > formation of alkyne

Question 3

what happens when HX or X2 added to alkyne

Answer 3

produces alkene

but reaction intermediate is not vinyl cation as it is highly unstable since no resonance stabilisation

forms cyclic intermediate instead (protonium ion) > cyclic structure involves a bridged proton interacting with the 2 carbons

Question 4

how to convert alkynes to carbonyl (mainly aldehydes)

Answer 4

via hydration of alkynes

1. Mercury (II) ion (cat. Hg(OAc)2) adds to more substituted C of alkyne > mercurninium ion
2. water attacks more positively charged C > enol intermediate
3. enol undergos tautomerisation (proton transfer and recognition of double bond) > aldehyde

Question 5

what happens during hydroboration of alkynes

Answer 5

1. reaction follows anti-maokovnikov’s rule (B adds to less substituted C) > organoborane intermediate
2. organoborane treated with H2O2 in NaOH > replace B with OH > enol > tautomerisation > ketone

Question 6

difference between hydration and hydroboration of alkynes

Answer 6

hydration of terminal alkynes > aldehydes, hydration of internal alkynes > ketones

hydroboration of alkynes > ketones for both terminal and internal

Question 7

reduction of alkynes

Answer 7

addition of H2 over metal catalyst converts alkynes to alkenes (complete reduction)

addition of first H2 > alkene > further reduction to alkane

Question 8

conversion of alkynes to CIS-alkenes

Answer 8

addition of H2 using Lindlar’s catalyst

Lindlar’s catalyst selectively reduce alkynes to cis-alkenes > prevent further reduction to alkane

Question 9

conversion of alkynes to TRANS-alkenes

Answer 9

alkali metals (LI, Na) dissolved in liquid NH3 > work as reducing agent > produce trans-alkenes

Question 10

how is acetylide anions formed from alkynes

Answer 10

C in alkyne is sp hybridised > more s character > C is more electronegative > can stabilise negative charge that forms when H is removed

terminal proton can be abstracted from strong base under anhydrous conditions

Question 11

how does acetylide anions create new C-C bond formation

Answer 11

they can react as nucleophiles and bases

reaction with primary alkyl halide produces new C-C

reaction only effective with primary alkyl bromide and alkyl iodides