Comparing Reactivity of Cyclohexane and Hexane

[davon806]

I have done several experiments on last week.The purpose of the experiment is to test the reactivity of the alkanes(combustion,substitution reaction,etc),using cyclohexane as an example.
My book said the reason of using cyclohexane is because it is cheaper and less hazardous to use than hexane.Why it is less hazardous than hexane?
Thx

 

[JohnRC]

I have done several experiments on last week.The purpose of the experiment is to test the reactivity of the alkanes(combustion,substitution reaction,etc),using cyclohexane as an example.
My book said the reason of using cyclohexane is because it is cheaper and less hazardous to use than hexane.Why it is less hazardous than hexane?
Thx

It is easy enough to check out the "whether" by downloading a MSDS, That does not really say "why".

The cyclohexane MSDS and the hexane MSDS look rather similar to me -- perhaps the toxicological aspects of hexane are slightly more suggestive of hazard, but the difference is fairly marginal.

The only suggestion I would have is that perhaps because cyclohexane has a fairly fixed shape while hexane can twist itself into different conformations more easily, the hexane can more easily find its way into places where it might interfere with biochemical mechanisms. But as neither molecule has hydrophilic (water-loving) groups this would not be a strong effect.

 

[chemisttree]

Cyclohexane has a higher boiling point. Safer.

 

[JohnRC]

Cyclohexane has a higher boiling point. Safer.

Of course that is it!

(Easy to forget about simple considerations)

 

[davon806]

But let's consider the boiling point of heptane or octane(the straight-chain one,I have forgetten the name),it is higher than that of cyclohexane,then why we don't use them?
Is it because heptane is more expensive?but why it is more expensive than hexane and cyclohexane?

 

[JohnRC]

But let's consider the boiling point of heptane or octane(the straight-chain one,I have forgetten the name),it is higher than that of cyclohexane,then why we don't use them?
Is it because heptane is more expensive?but why it is more expensive than hexane and cyclohexane?

The idea that is expressed in this post is quite a sound and good one.

There are "straight chain" hydrocarbons of every chain length. Each of them is often indicated by "n-" in front of the name, e.g. "n-decane". Boiling points steadily increase as the chain length increases:

pentane: 36°C
hexane: 69°C
heptane: 98°C
n-octane: 126°C

cyclohexane boils at 81°C.

So the reason why not is because heptane is more expensive. Why?

Cyclohexane is a product that has a major industrial consumption. It is a reactant in the synthesis of adipic acid, one of the two reactants needed in a very pure state for preparation of nylon(6,6). It is also used in the synthesis of caprolactam, which is the starting material for polymerization to nylon(6). Because it is manufactured and used on a large scale, it is relatively cheap.

Hexane as an industrial chemical is mainly used as a solvent rather than a reagent, and is often used as a mixture of isomers, much cheaper than pure n-hexane. There are, incidentally, recent concerns about probable long term toxicity of n-hexane because it is metabolized to 2,5-hexadione --CH₃C=OCH₂CH₂C=OCH₃ (among other things).

Hexane is obtained directly from the oil refining processes. The name usually refers to a fraction which a) is unreactive (alkenes and aromatics are removed), and b) has a narrow boiling range around 70°C. It therefore might contain significant and fairly major amounts of other hexane isomers, and small amounts of pentanes and heptanes. If pure n-hexane is required for any particular application the price rises fairly steeply with the degree of refinement.

Oil refining fractions with higher boiling points than hexane are usually sold as "petroleum spirit" in 20°C boiling ranges, e.g. "petroleum spirit 80-100" or "petroleum spirit 140-160", or as mixtures of isomers "octane" or "decane", etc. "distillate", "heating oil", "kerosene" are also names given to higher boiling fractions.

A more specific hydrocarbon isomer, like n-heptane, can be a very expensive material, especially if there is not a large industrial demand for that particular isomer.

 

[chemisttree]

Without knowing exactly what reactions you have performed on the cyclohexane we have no way to accurately answer your question. Cyclohexane is safer than hexanes in combustion analysis. Mixed alkane isomers might give you fits if you are looking at substitution reactions though. Cyclohexane contains only secondary carbons whereas isooctane has primary, secondary and tertiary carbons. Might be complicated to study substitution reactions for complex mixtures of species containing primary, secondary and tertiary carbons, no?