Answers

2016-03-29T10:58:20+05:30
Nomenclature: 
Functional group suffix = -yne (review
Disubstituted alkynes, R-CºC-R', are described as "internal" alkynes 
Monosubstituted alkynes, R-CºC-H, are described as "terminal" alkynes. 

As with alkenes, the more highly substituted internal alkynes are more stable.By comparing thermodynamic data of alkynes and alkenes, it can be seen that the "extra" p bond in an alkyne is weaker than the alkene p bond:DHh 1-hexyne = 290 kJ/mol (69.2 kcal/mol) vs 1-hexene = 126 kJ/mol (30.2 kcal/mol)
So CºC  to C=C = 164 kJ/mol (39 kcal/mol) while C=C to C-C = 126 kJ/mol (30.2 kcal/mol) 
Therefore the "extra"  p bond is 38 kJ/mol (8.8 kcal/mol) weaker that an alkene p bond.
Rank 1-hexyne and 3-hexyne for each of the following properties:(a) heat of hydrogenation   (c) heat of formation (b) heat of combustion  (d) stability

Structure:

The alkyne functional group consists of two sp hybridised C atoms bonded to each other via one s and two p bonds.The 2 p bonds are produced by the side-to-side overlap of the two pairs of p-orbitals not utilised in the hybrids.The substituents are attached to the CºC via sigma bonds.The 2 C of the CºC and the 2 atoms attached directly to the CºC are linear.Since alkynes are linear, they cannot exist as cis- trans- isomers.
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