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Chapter 9 Alkynes. 9.1 Sources of Alkynes. +. H 2. +. H 2. HC. CH. CH 2. H 2 C. Acetylene. Industrial preparation of acetylene is by dehydrogenation of ethylene. 800°C. CH 2. H 2 C. CH 3 CH 3. 1150°C. cost of energy makes acetylene a more

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acetylene
+

H2

+

H2

HC

CH

CH2

H2C

Acetylene
  • Industrial preparation of acetylene isby dehydrogenation of ethylene

800°C

CH2

H2C

CH3CH3

1150°C

cost of energy makes acetylene a more

expensive industrial chemical than ethylene

nomenclature
Acetylene and ethyne are both acceptableIUPAC names for

HC

CH

HC

CCH3

HC

CCH2CH3

Propyne

(CH3)3CC

CCH3

Nomenclature

Higher alkynes are named in much the sameway as alkenes except using an -yne suffixinstead of -ene.

1-Butyne

4,4-Dimethyl-2-pentyne

9 3 physical properties of alkynes
9.3Physical Properties of Alkynes

The physical properties of alkynes are similar to those of alkanes and alkenes.

structure
120 pm

H

C

C

H

106 pm

106 pm

Structure
  • linear geometry for acetylene

121 pm

C

CH3

C

H

146 pm

106 pm

cycloalkynes
Cycloalkynes
  • Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperaturefor a reasonable length of time.
  • Cyclooctyne polymerizeson standing.
sp hybridization in acetylene
sp Hybridization in Acetylene

Mix together (hybridize) the 2s orbital and one of the three 2p orbitals

2p

2p

2sp

2s

sp hybridization in acetylene1
sp Hybridization in Acetylene

Mix together (hybridize) the 2s orbital and one of the three 2p orbitals

2p

Each carbon has two half-filled sp orbitalsavailable to form s bonds.

2sp

s bonds in acetylene
s Bonds in Acetylene

Each carbon isconnected to ahydrogen by as bond. The twocarbons are connectedto each other by as bond and two p bonds.

Figure 9.2 (a)

p bonds in acetylene
p Bonds in Acetylene

One of the twop bonds in acetylene isshown here.The second pbond is at rightangles to the first.

Figure 9.2 (b)

p bonds in acetylene1
p Bonds in Acetylene

This is the secondof the twop bonds in acetylene.

Figure 9.2 (c)

figure 9 3 electrostatic potential in acetylene
Figure 9.3 Electrostatic Potential in Acetylene

The region of highest negative charge encirclesthe molecule around itscenter in acetylene.

The region of highest negative charge lies aboveand below the molecular plane in ethylene.

table 9 1 comparison of ethane ethylene and acetylene
Ethane Ethylene Acetylene

C—C distance

153 pm

134 pm

120 pm

C—H distance

111 pm

110 pm

106 pm

H—C—C angles

111.0°

121.4°

180°

C—C BDE

368 kJ/mol

611 kJ/mol

820 kJ/mol

C—H BDE

410 kJ/mol

452 kJ/mol

536 kJ/mol

hybridization of C

sp3

sp2

sp

% s character

25%

33%

50%

pKa

62

45

26

Table 9.1Comparison of Ethane, Ethylene, and Acetylene
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