Chapter 9 Alkynes

Question 0

terminal (monosubstituted) alkyne

Answer 0

compounds with triple bond at the end of the carbon chain

Question 1

alkyne

Answer 1

triple bond between carbons

Question 2

disubstituted alkynes

Answer 2

carbon carbon triple bonds are internal

Question 3

most distinctive aspect of acetylene and terminal alkynes

Answer 3

their acidity

-most acidic of all hydrocarbons

Question 4

dehydration of ethylene

Answer 4

• leads to acetylene and H2 gas

- reaction is endothermic

Question 5

fatty acids

Answer 5

carboxylic acids with unbranched chains of 12-20 carbon atoms

Question 6

nomenclature of alkynes

Answer 6

-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)

Question 7

physical properties of alkynes

Answer 7

• low density
• low water-solubility
• boiling points similar to alkanes

Question 8

hybridization of alkynes

Answer 8

sp hybridization with angles of 180 degrees between carbons (linear)

Question 9

structural features of alkanes, alkenes, alkynes

Answer 9

• 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

Question 10

s character in C-H bonds

Answer 10

sp3 (25%), sp2 (33%), sp (50%)

Question 11

sp character in acetylene

Answer 11

• 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+

Question 12

carbanion

Answer 12

-conjugate base of a hydrocarbon
-ex. HC~CH –> HC~C:
CH4 –> CH3:

Question 13

acid vs base

Answer 13

• strong acid has a weak conjugate base

- weak acid has a strong conjugate base

Question 14

lower pKa

Answer 14

stronger acid

Question 15

acetylene

Answer 15

-a very strong acid in terms of hydrocarbons, therefore it has a weak conjugate base

Question 16

electronegativity of carbon

Answer 16

-increases with its s character (sp3<sp)

Question 17

hydrocarbon acidity

Answer 17

-increases with its s character

Question 18

anions of acetylene and terminal alkynes

Answer 18

-are nuclephilic and react with methyl and primary alkyl halides to form C-C bonds by nucleophilic substitution

Question 19

carbon-carbon bond forming reactions

Answer 19

-are used to prepare alkynes

Question 20

alkynation of acetylene

Answer 20

-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

Question 21

alkynation of acetylene

Answer 21

-synthesis is via SN2 but requires either a methyl or or a primary alkyl halide

Question 22

double dehydrohalongenation

Answer 22

preparation of alkynes by elimination reactions

Question 23

geminal dihalide

Answer 23

halide in which both halogens are on the same carbon
H X
R-C-C-R
H X

Question 24

vicinal dihalide

Answer 24

halide in which the halogens are on adjacent carbons
H H
R-C-C-R
X X

Question 25

elimination reaction to form alkyne

Answer 25

-geminal dihalide requires 2 NaNH2 to remove both halides and 1 NaNH2 to complete the reaction to form an alkyne

Question 26

hydrogenation of alkynes

Answer 26

• 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)

Question 27

heat of hydrogenation

Answer 27

• alkyne is greater than twice the heat of hydrogenation of an alkene
• higher in terminal alkynes compared to internal alkynes

Question 28

metal-ammonia reduction of alkynes

Answer 28

• must use a group 1 metal (Li, Na, K) and liquid ammonia (NH3)
• converts to trans alkenes

Question 29

addition of hydrogen halides to alkynes

Answer 29

• 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)

Question 30

HBr in the presence of peroxides

Answer 30

• addition of hydrogen halides

- is opposite Markovnikov’s rule (H adds to C with fewer H’s)

Question 31

enol

Answer 31

• 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

Question 32

tautomers

Answer 32

-constitutional isomers that equilibrate by migration of an atom or a group

Question 33

tautomerism

Answer 33

-equilibration of constitutional isomers

Question 34

ketones

Answer 34

-more stable than their enol precursors

Question 35

hydration of alkynes

Answer 35

• 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)

Question 36

addition of halogens to alkynes

Answer 36

• 2 molecules of halogen (2 Cl2) are need to yield tetrahaloalkanes
• dihaloalkene is intermediate with anti addition of halogen

Question 37

bronsted base

Answer 37

• proton acceptor

- picks up another H