 |
Organic Chemistry 4e Carey | |
|---|
Online Learning Center
|
 |
Chapter 9: Alkynes
Summary |
Alkynes |
Preparation of Alkynes |
Addition Reactions of Alkynes |
Self Assessment |
Quiz |
|
Alkynes
|
Chapter 9 :
Alkynes
|
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.
| Classify
each of the following as an internal or a terminal alkyne: |
(a) 1-hexyne  |
(c) cyclooctyne |
| (b) 3-octyne |
(d) propyne |
Stability:
- 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.
|
 |
 |
|
|
|
The two separate
perpendicular p molecular orbitals
|
|
Combined p molecular
orbitals |
Physical
Properties:
As with hydrocarbons
in general, alkynes are non-polar and are insoluble in water but soluble in
non-polar organic solvents.
Reactivity:
- The p bonds
are a region of high electron density (red)
so alkynes are typically nucleophiles.
- Alkynes typically
undergo addition reactions in which one or both of the p-bonds are
converted to new s bonds.
- Terminal alkynes,
R-CºC-H, are quite acidic (indicated by blue)
for hydrocarbons, pKa = 26
- Deprotonation
of a terminal acetylene gives an acetylide ion.The
acetylide ion is a good nucleophile and can be alkylated to give higher
alkynes.
|
 |
Acidity
of Terminal Alkynes
Summary
Terminal alkynes are unusual
for simple hydrocarbons in that they can be deprotonated (pKa = 26) using an
appropriate base (typically NaNH2, pKa = 36) to generate a carbanion
which can function as a C centered nucleophile and so allow for the formation
of new C-C bonds by reacting with C centered electrophiles (such
as alkyl halides).
In order to appreciate what
makes the terminal alkyne more acidic than most other hydrocarbons, we should
look at the stability of the conjugate base (i.e. the carboanion).
For each type of carbanion
shown, the nature of the hybrid orbital containing the electron pair is important.
Increased s character (sp = 50%, sp2 = 33% and
sp3 = 25%) implies that the alkyne sp orbital is closer
to the nucleus and so there is greater electrostatic stabilisation of the electron
pair. Therefore the conjugate base of the alkyne is the most stable and the
most readily formed.
However the terminal alkyne C-H bond is not strongly acidic and a strong base,
such as the amide ion, NH2-, is required to form the carbanion.
Could you use a base such
as NaOH or NaOEt for this reaction ?
Related reactions
- Alkylation of terminal
alkynes
Begin a search: Catalog | Site | Campus Rep
MHHE Home | About MHHE | Help Desk | Legal Policies and Info | Order Info | What's New | Get Involved
Copyright ©2000 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms
of Use and Privacy
Policy.
McGraw-Hill Higher Education is one of the many fine businesses of The McGraw-Hill Companies.
For further information about this site contact mhhe_webmaster@mcgraw-hill.com.
