|Chapter 9 : Alkynes|
|Qu 1:||The π bond in an alkene is given by : C=C - C-C|
(146 kcal/mol) - 368 kJ/mol (88 kcal/mol) = 243
kJ/mol (58 kcal/mol)
|The second π bond in an alkyne is given by : C≡C - C=C|
(196 kcal/mol) - 611 kJ/mol (146 kcal/mol) = 209 kJ/mol (50
This indicates that it should be recognised that the extra π bond in an alkyne is a weaker bond than the p bond in an alkene.
|Qu 2:||Consider the following points about the reaction of 2-butyne with HBr|
|(a) Since the reaction gives predominantly the 2-bromo-2-butene, the product must be less reactive than the alkyne starting material otherwise a mixture the mono-bromide and the dibromide would be formed.|
Protonation of 2-bromo-2-butene gives a secondary carbocation regardless of which of the two C get protonated.
However, since the only product is 2,2-dibromobutane, the intermediate must be that from the carbocation with the +ve charge formed adjacent to the bromine.
This can be justified by considering the resonance stabilising influence the bromine can provide.
|Qu 3:||The final product is 2-butyne.|
|The steps are: (i) propene to 1,2-dibromopropane (addition) then (ii) elimination to propyne, a terminal alkyne, and (iii) removal of the terminal H gives the carbanion which undergoes an SN2 with the methyl iodide forming a new CC bond.|
|Qu 4:||The major products are:|
|© Dr. Ian Hunt, Department of Chemistry|