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Rules of Thumb for Organic and Inorganic Chemicals

General Rules of Thumb for Physical Properties of Organic Chemicals
Covalent bonds join two of the same atoms or atoms of similar electro-negativity and are therefore considered non-polar or very slightly polar and these covalent bonds are very weak. Molecules held together by such covalent bonds include alkanes, alkenes, alkynes, and cyclic aliphatic hydrocarbon. These weak forces occur between molecules in close proximity to each other or on small surfaces. Large molecules with large surfaces, therefore, have stronger intermolecular forces. As such, the boiling points and melting points rise with an increase in the number of carbon atoms the molecule contains by 20 to 30 degrees Celsius for each carbon on the chain as seen in the table below. Branched isomers have a lower boiling point compared to the straight chain. Cyclic aliphatic hydrocarbons properties are similar to those of the corresponding open-chain hydrocarbons. Alkanes, alkenes, alkynes, and cyclic aliphatics are soluble in non-polar solvents such as benzene and are insoluble in water. Nearly all organic compounds are less dense than water. In general, to be denser than water a compound must contain a heavy atom like bromine or iodine, or several atoms like chlorine.
Name Melting Point deg. C. Boiling Point deg. C. Density at 20 deg. C.

Methane

-183

-162

gas

Ethane

-172

-88.5

gas

Ethylene

-169

-102

gas

Acetylene

-82

-75

gas

Propane

-187

-42

gas

Propylene

-185

-48

gas

Propyne

-101.5

-75

gas

Cyclopropane

-127

-33

gas

n-Butane

-138

0

gas

Isobutane

-159

-12

gas

1-Butene

-6.5

gas

cis-2-Butene

-139

4

gas

trans-2-Butene

-106

1

gas

Isobutylene

-141

-7

gas

1-Butyne

-122

9

gas

Cyclobutane

-80

13

gas

2-Butyne

-24

27

0.694

n-Pentane

-130

36

0.626

Isopentane

-160

28

0.620

1-Pentene

30

0.643

cis-2-Pentene

-151

37

0.655

trans-2-Pentene

36

0.647

1-Pentyne

-98

40

0.695

2-Pentyne

-101

55

0.714

Cyclopentane

-94

49

0.746

n-Hexane

-95

69

0.659

1-Hexene

-138

63.5

0.675

1-Hexyne

-124

72

0.719

Cyclohexane

6.5

81

0.788

n-Heptane

-90.5

98

0.684

1-Heptene

-119

93

0.698

1-Heptyne

-80

100

0.733

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