Organic Chemistry

PROBLEM 3

Draw the structures and give the common and systematic names for the seven alkynes with

molecular formula

PROBLEM 4

Give the systematic name for each of the following compounds:

a. c.

b. d.

PROBLEM 5

Which would you expect to be more stable, an internal alkyne or a terminal alkyne? Why?

6.2 Physical Properties of

Unsaturated Hydrocarbons

All hydrocarbons have similar physical properties. In other words, alkenes and

alkynes have physical properties similar to those of alkanes (Section 2.9). All are in-

soluble in water and all are soluble in solvents with low polarity such as benzene and

ether. They are less dense than water and, like other homologous series, have boiling

points that increase with increasing molecular weight (Table 6.1). Alkynes are more

linear than alkenes, and a triple bond is more polarizable than a double bond

(Section 2.9). These two features cause alkynes to have stronger van der Waals inter-

actions. As a result, an alkyne has a higher boiling point than an alkene containing the

same number of carbon atoms.

Internal alkenes have higher boiling points than terminal alkenes. Similarly, inter-

nal alkynes have higher boiling points than terminal alkynes. Notice that the boiling

point of cis-2-butene is slightly higher than that of trans-2-butene because the cis iso-

mer has a small dipole moment, whereas the dipole moment of the trans isomer is zero

(Section 3.4).

CH 3 CH 2 CHC CH

CH 2 CH 2 CH 3

CH 3 CH 2 CHC CCH 2 CHCH 3

Br Cl

BrCH 2 CH 2 C CCH 3 CH 3 OCH 2 C CCH 2 CH 3

C 6 H 12.

240 CHAPTER 6 Reactions of Alkynes • Introduction to Multistep Synthesis

Table 6.1 Boiling Points of the Smallest Hydrocarbons

bp (°C) bp (°C) bp (°C)

ethane ethene ethyne

propane propene propyne

8

butane 1-butene 1-butyne

36.1 30 39

pentane 1-pentene 1-pentyne

68.7 63.5 71

hexane 1-hexene 1-hexyne

3.7 27

cis-2-butene 2-butyne

0.9 55

trans-2-butene 2-pentyne

CH 3 CH“CHCH 3 CH 3 CH 2 C‚CCH 3

CH 3 CH“CHCH 3 CH 3 C‚CCH 3

CH 3 (CH 2 ) 4 CH 3 CH 3 CH 2 CH 2 CH 2 CH“CH 2 CH 3 CH 2 CH 2 CH 2 C‚CH

CH 3 (CH 2 ) 3 CH 3 CH 3 CH 2 CH 2 CH“CH 2 CH 3 CH 2 CH 2 C‚CH

CH 3 CH 2 CH 2 CH 3 - 0.5 CH 3 CH 2 CH“CH 2 - 6.5 CH 3 CH 2 C‚CH

CH 3 CH 2 CH 3 - 42.1 CH 3 CH“CH 2 - 47 CH 3 C‚CH - 23

CH 3 CH 3 - 88.6 H 2 C“CH 2 - 104 HC‚CH - 84

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