What are the Physical properties of Alkenes?

Aug 26, 2022, 16:45 IST

Physical properties of the alkanes

Physical states: Due to the weak van der Waals forces, the first four normal alkanes (C₁ to C₄) are colourless and odourless gases, the next thirteen normal alkanes (C₅ to C₁₇) are colourless and odourless liquids, and from C₁₈ onwards, colourless and odourless solids at 298K and 1 atm pressure.

Boiling points: The boiling points of the normal (unbranched) alkanes show a regular (steady) increase with increasing molecular mass. Except for the very small alkanes (C₁ to C₄), the boiling point rises 20 to 30 degrees for each carbon that is added to the chain.

Melting points: The normal alkanes do not exhibit the same smooth increase in melting points with increasing molecular mass that they show in their boiling points. There is an alternation as one moves from a normal alkane with an even number of carbon atoms to the next one with an odd number of carbon atoms. For example, n –propane (mp – 187⁰C) melts lower than ethane (mp – 172⁰C) and also lower than methane (mp – 183⁰C). n – Butane (mp – 138⁰C) melts 49⁰C higher than propane but only 8⁰C lower than pentane (mp – 130⁰C). Hence there is a saw-tooth pattern. If, however, the even-and odd-numbered alkanes are plotted on separate curves, there is a smooth increase in melting point with increasing molecular mass.

Effect of branching on boiling and melting points: In a group of isomeric alkanes, the normal compound always has the highest b.p. and m.p. Generally, the greater the branching, the lower is the b.p. Thus n-butane has a boiling point of 0⁰C and isobutane -12⁰C. n-Pentane has a boiling point of 36⁰C, isopentane with a single branch 28⁰C, and neopentane with two branches 9.5⁰C. Similarly n-hexane boils at 68.7⁰C, and 2-methyl pentane and 3-methylpentane, each having one branch, boil lower at 60.3 and 63.3⁰C, respectively. 2, 3– Dimethylbutane and 2, 2-dimethylbutane each with two branches boil lower, at 58 and 49.7⁰C, respectively.

The effect of chain branching on the melting points of alkanes is not easy to predict. However, the chain-branching producing highly symmetrical structures results in abnormally high melting points. For example, the compound 2, 2, 3, 3 – tetramethylbutane (mp 100.7⁰C) melts 43.7⁰C higher than n-octane (mp - 57⁰C) but boils 20⁰C lower than it.

Solubility: Alkanes are almost completely insoluble in water. Liquid alkanes are soluble in one another and they generally dissolve in non-polar solvents and solvents of low polarity. Examples of good solvents for alkanes are benzene, carbon tetrachloride, chloroform and ether. The solubility diminishes with increase in molecular mass.

Density: As a class, the alkanes are the least dense of all groups of organic compounds. The density also increases with molecular mass of the alkanes, but tends to level off at about 0.79 g mL^-¹. Thus all alkanes are considerably less dense than water (1.00 g mL^-¹, the density of water at 4⁰C). As a result, petroleum (a mixture of hydrocarbons) floats on water.

General chemical properties of the alkanes

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The alkanes are sometimes referred to by the old- fashioned name of paraffins. This name arose through contracting the two Latin words ‘parum affins’ which implies little affinity or not enough affinity. This name was suggested because alkanes were apparently very unreactive. But reactivity depends upon the choice reagent. Under ‘ordinary’ conditions, the alkanes are inert toward reagents such as acids, alkalis, oxidizing reagents, reducing reagents, etc., but are reactive if the right conditions are used.

Alkanes are inert toward hydrochloric and sulfuric acids, they react readily with acids like HF-SbF₅ and FSO₃H-SbF₅ (“magic acid”) to yield a variety of products. Alkanes are inert toward oxidizing agents like potassium permanganate or sodium dichromate, they get oxidized by halogens. Certain yeasts feed happily on alkanes to produce proteins.