7.2: Physical Properties and Important Common Names

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Learning Objectives

• memorize the common names for vinylic and allylic groups including isoprene and styrene
• predict the relative physical properties of alkenes

Common names

The carbon atoms sharing the double bond can be referred to as the "vinyl carbons". The carbon atoms adjacent to the vinyl carbon atoms are called "allylic carbons". These carbon atoms have unique reactivity because of the potential for interaction with the pi bond.

Overall, common names remove the -ane suffix and add -ylene. There are a couple of unique ones like ethenyl's common name is vinyl and 2-propenyl's common name is allyl that need to be memorized.

• vinyl substituent H2C=CH-
• allyl substituent H2C=CH-CH2-
• allene molecule H2C=C=CH2
• isoprene is shown below

Physical Properties of Selected Alkenes

Some representative alkenes—their names, structures, and physical properties—are given in the table below.

Physical Properties of Some Selected Alkenes
IUPAC Name Molecular Formula Condensed Structural Formula Melting Point (°C) Boiling Point (°C)
ethene C2H4 CH2=CH2 –169 –104
propene C3H6 CH2=CHCH3 –185 –47
1-butene C4H8 CH2=CHCH2CH3 –185 –6
1-pentene C5H10 CH2=CH(CH2)2CH3 –138 30
1-hexene C6H12 CH2=CH(CH2)3CH3 –140 63
1-heptene C7H14 CH2=CH(CH2)4CH3 –119 94
1-octene C8H16 CH2=CH(CH2)5CH3 –102 121

Polarity and Physical Properties

Alkenes are non-polar hydrocarbons. The dominant intermolecular forces shared by alkenes are the London dispersion forces. These interactions are weak and temporary, so they are easily disrupted.

Physical States: The physical states reflect the weak attractive forces between molecules. Ethene, propene, and butene exist as colorless gases. Alkenes with 5 to 14 carbons are liquids, and alkenes with 15 carbons or more are solids.

Density: Alkenes are less dense than water with most densities in the range of 0.6 to 0.7 g/mL. Alkenes float on top of water.

Solubility: Alkenes are virtually insoluble in water, but dissolve in organic solvents. The reasons for this are exactly the same as for the alkanes.

Boiling Points: The boiling point of each alkene is very similar to that of the alkane with the same number of carbon atoms. Boiling points of alkenes depend on more molecular mass (chain length). The more intermolecular mass is added, the higher the boiling point. Intermolecular forces of alkenes gets stronger with increase in the size of the molecules. In each case, the alkene has a boiling point which is a small number of degrees lower than the corresponding alkane. The only attractions involved are Van der Waals dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains.

 Compound Boiling points (oC) Ethene -104 Propene -47 Trans-2-Butene 0.9 Cis-2-butene 3.7 Trans 1,2-dichlorobutene 155 Cis 1,2-dichlorobutene 152 1-Pentene 30 Trans-2-Pentene 36 Cis-2-Pentene 37 1-Heptene 115 3-Octene 122 3-Nonene 147 5-Decene 170

Melting Points: Melting points of alkenes depends on the packaging of the molecules so the stereochemistry of the carbon-carbon double bond has a strong influence on the relative melting points. Alkenes have similar melting points to that of alkanes, however, in cis isomers molecules are package in a U-bending shape, therefore, will display a lower melting points to that of the trans isomers. This effect is notable when comparing the melting points of fats and oils. The differences in the melting points is strongly influenced by the long hydrocarbon tails. Oils have a greater number of cis double bonds and exist as liquids at room temperature. Whereas, fats are primarily saturated and exist as solids at room temperature.

 Compound Melting Points (0C) Ethene -169 Propene -185 Butene -138 1-Pentene -165 Trans-2-Pentene -135 Cis-2-Pentene -180 1-Heptene -119 3-Octene -101.9 3-Nonene -81.4 5-Decene -66.3

Exercise

1. Draw the bond-line structures for the following compounds in order of increasing boiling point: 2-methyl-2-pentene; 2-hexene; isoprene; 2-heptene.

2. Which phase will contain the most 3-octene?

a) water or hexane

b) water or benzene

c) methanol or 1-octanol

1. relative boiling points

2. a) hexane (Hydrocarbons are hydrophobic and lipophilic.)

b) benzene (Hydrocarbons are hydrophobic and lipophilic.)

c) 1-octanol (Hydrocarbons seek the solvent with the most carbons and fewest polar groups.)

Contributors

• Trung Nguyen
• Jim Clark (Chemguide.co.uk)
• Layne A. Morsch (University of Illinois Springfield)

7.2: Physical Properties and Important Common Names is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.