Chapter 9 Alkynes Flashcards
terminal (monosubstituted) alkyne
compounds with triple bond at the end of the carbon chain
alkyne
triple bond between carbons
disubstituted alkynes
carbon carbon triple bonds are internal
most distinctive aspect of acetylene and terminal alkynes
their acidity
-most acidic of all hydrocarbons
dehydration of ethylene
- leads to acetylene and H2 gas
- reaction is endothermic
fatty acids
carboxylic acids with unbranched chains of 12-20 carbon atoms
nomenclature of alkynes
-ane is replaced by -yne
if there is a double bond and a triple bond in compound, which ever bond has the lowest number is named first )ties are broken in favor of the double bond)
physical properties of alkynes
- low density
- low water-solubility
- boiling points similar to alkanes
hybridization of alkynes
sp hybridization with angles of 180 degrees between carbons (linear)
structural features of alkanes, alkenes, alkynes
- geometry changes from tetrahedral->trigonal planar->linear
- the C-C and C-H bonds become shorter and stronger
- the acidity of the C-H increases
- sp3->sp2->sp hybridization states
s character in C-H bonds
sp3 (25%), sp2 (33%), sp (50%)
sp character in acetylene
- electrons are in a sp orbital which means that they are held more tightly to the nucleus
- this means that electrons do not have the freedom to fool around with H+
carbanion
-conjugate base of a hydrocarbon
-ex. HC~CH –> HC~C:
CH4 –> CH3:
acid vs base
- strong acid has a weak conjugate base
- weak acid has a strong conjugate base
lower pKa
stronger acid
acetylene
-a very strong acid in terms of hydrocarbons, therefore it has a weak conjugate base
electronegativity of carbon
-increases with its s character (sp3<sp)
hydrocarbon acidity
-increases with its s character
anions of acetylene and terminal alkynes
-are nuclephilic and react with methyl and primary alkyl halides to form C-C bonds by nucleophilic substitution
carbon-carbon bond forming reactions
-are used to prepare alkynes
alkynation of acetylene
-acetylene is converted to its conjugate base by sodium amide (NaNH2)
HC~CH + NaNH2 –> HC~CNa + NH3
-SN2 an alkyl halide is added, HC~CNa acts as a nucleophile, displacing halide
HC~CNa + CH3CH2Br –> HC~CCH2CH3 + NaBr
alkynation of acetylene
-synthesis is via SN2 but requires either a methyl or or a primary alkyl halide
double dehydrohalongenation
preparation of alkynes by elimination reactions
geminal dihalide
halide in which both halogens are on the same carbon
H X
R-C-C-R
H X
vicinal dihalide
halide in which the halogens are on adjacent carbons
H H
R-C-C-R
X X
elimination reaction to form alkyne
-geminal dihalide requires 2 NaNH2 to remove both halides and 1 NaNH2 to complete the reaction to form an alkyne
hydrogenation of alkynes
- 2 H2 are added to triple bond in the presence of a metal plate to reduce triple bond to a single bond
- syn addition (cis compound)
heat of hydrogenation
- alkyne is greater than twice the heat of hydrogenation of an alkene
- higher in terminal alkynes compared to internal alkynes
metal-ammonia reduction of alkynes
- must use a group 1 metal (Li, Na, K) and liquid ammonia (NH3)
- converts to trans alkenes
addition of hydrogen halides to alkynes
- Markovnikov’s rule: H adds to the C with more H’s, halide adds to the C with fewer H’s
- must have 2 molecules of the hydrogen halide
- rate law = k[alkyne][HX]^2
- alkane produced is a germinal halide (both halides are attached to same carbon)
HBr in the presence of peroxides
- addition of hydrogen halides
- is opposite Markovnikov’s rule (H adds to C with fewer H’s)
enol
- an alcohol in which the hydroxyl group is attached to the C=C
- isomerization to aldehydes or ketones is rapid under the condition of their formation
tautomers
-constitutional isomers that equilibrate by migration of an atom or a group
tautomerism
-equilibration of constitutional isomers
ketones
-more stable than their enol precursors
hydration of alkynes
- uses sulfuric acid as the reaction medium and Hg2+ as a catalyst
- follows Markovnikov’s rule (H’s add to C with more H’s)
addition of halogens to alkynes
- 2 molecules of halogen (2 Cl2) are need to yield tetrahaloalkanes
- dihaloalkene is intermediate with anti addition of halogen
bronsted base
- proton acceptor
- picks up another H
