Alkynes. CnH2n-2

C2H2 H:C:::C:H H—C C—H sp => linear, 180o

acetylene

ethyne

C3H4 CH3CCH methylacetylene

propyne

nomenclature:

common names: “alkylacetylene”

IUPAC: parent chain = longest continuous carbon chain that contains the triple bond.

alkane drop –ane add -yne

prefix locant for the triple bond, etc.

CH3CH2CCCH3 2-pentyne

ethylmethylacetylene

“terminal” alkynes have the triple bond at the end of the chain:

CH3

CH3CH2CCH HCCCHCH2CH3

1-butyne 3-methyl-1-pentyne

ethylacetylene sec-butylacetylene

physical properties:

weakly or non-polar, no H-bonding

relatively low mp/bp

water insoluble

Synthesis, alkynes:

1. dehydrohalogenation of vicinal dihalides

H H H | | |— C — C — + KOH — C = C — + KX + H2O | | | X X X

H | — C = C — + NaNH2 — C C — + NaX + NH3

| X

H H | | — C — C — + 2 KOH — C C — + KX + H2O | | heat X X

CH3CH2CHCH2 + KOH; then NaNH2 CH3CH2CCH Br Br

“ + 2 KOH, heat

alkene vicinal dihalide alkyneX2 1. KOH

2. NaNH2

CH3CH=CH2 CH3CHCH2 CH3CCH

Br Br

Br2 1. KOH

2. NaNH2

Synthesis of propyne from propane

CH3CH2CH3

Br2, heatCH3CH2CH2-Br + CH3CHCH3

Br

KOH(alc)

CH3CH=CH2Br2

CH3CHCH2

Br Br

KOH

CH3CH CH

Br

NaNH2CH3C CH

2. coupling of metal acetylides with 1o/CH3 alkyl halides

R-CC-Na+ + R´X R-CC-R´ + NaX

a) SN2

b) R´X must be 1o or CH3X

CH3CC-Li+ + CH3CH2-Br CH3CCCH2CH3

CH3C C Na + CH3CCH3

CH3

Br

CH3C CH

+

CH3C CH2

CH3

3o alkyl halide

E2 elimination!

note: R-X must be 1o or CH3 to get SN2!

some

alkynes

acids

bases

metals

oxid.

reduct.

halogens

terminal only

terminal only

Reactions, alkynes:

1. addition of H2 (reduction)

2. addition of X2

3. addition of HX

4. addition of H2O, H+

5. as acids

6. Ag+

7. oxidation

1. Addition of H2

H H | |— C C — + 2 H2, Ni — C — C — | | H H

alkane

requires catalyst (Ni, Pt or Pd)

HCCH + 2 H2, Pt CH3CH3

[ HCCH + one mole H2, Pt CH3CH3 + CH2=CH2 + HCCH ]

H \ /

Na or Li C = C anti- NH3(liq) / \

H— C C —

\ / H2, Pd-C C = C syn-

Lindlar catalyst / \ H H

CH3 H

\ /Na or Li C = C anti-

NH3(liq) / \ H CH3

trans-2-buteneCH3CCCH3

H H \ /

H2, Pd-C C = C syn- Lindlar catalyst / \ CH3 CH3

cis-2-butene

2. Addition of X2

X X X | | |— C C— + X2 — C = C — + X2 — C — C

— | | | X X X

Br Br BrCH3CCH + Br2 CH3C=CH + Br2 CH3-C-CH Br Br Br

3. Addition of hydrogen halides:

H H X | | |— C C— + HX — C = C — + HX — C — C — | | | X H X

a) HX = HI, HBr, HClb) Markovnikov orientation

ClCH3CCH + HCl CH3C=CH2 + HCl CH3CCH3

Cl Cl

4. Addition of water. Hydration.

O

— C C — + H2O, H+, HgO — CH2 — C—

H OH

— C = C —

“enol” keto-enol tautomerism

Markovnikov orientation.

CH3CH2CCH + H2O, H2SO4, HgO

1-butyne

O

CH3CH2CCH3

2-butanone

5. As acids. terminal alkynes only!

a) with active metals

CH3CCH + Na CH3CC-Na+ + ½ H2

b) with bases

CH3CCH + CH3MgBr CH4 + CH3C CMgBr

SA SB WA WB

acid strength:

CH4 < NH3 < HCCH < ROH < H2O < HF

HC CH + NaOH NR ( H2O = stronger acid! )

CH3CH2CCH + LiNH2 NH3 + CH3CH2CC-Li+

SA WA

6. Ag+ terminal alkynes only!

CH3CH2CCH + AgNO3 CH3CH2CC-Ag+

CH3CCCH3 + AgNO3 NR (not terminal)

formation of a precipitate is a test for terminal alkynes.

7. Oxidation

KMnO4

R-CC-R´ hot KMnO4 RCOOH + HOOCR´

carboxylic acids

O3; then Zn, H2O

CH3CH2CCCH3 + KMnO4

CH3CCH + hot KMnO4

CH3CCCH3 + O3; then Zn, H2O

CH3CH2COOH + HOOCCH3

CH3COOH + CO2

2 CH3COOH

Alkynes

Nomenclature

Syntheses

1. dehydrohalogenation of vicinal dihalide

2. coupling of metal acetylides with 1o/CH3X

Reactions, alkynes:

1. addition of H2 (reduction)

2. addition of X2

3. addition of HX

4. addition of H2O, H+

5. as acids

6. Ag+

7. oxidation