I have heard a theory from someone and just wanted to confirm if its true or not. Is it true that more bulkier an organic compound is, more likely it is to react? So does this mean that higher the molecular weight of an organic compound, more will be the probability of it to react?

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    $\begingroup$ Generally bulky substituents are working the other way round. $\endgroup$ – Mithoron Aug 13 '15 at 10:56
  • $\begingroup$ This is actually an interesting question but needs some work. Reaction rates are functions of collision probability. Collision probability depends on many factors; the commonly known factor is concentration but it would also depend on molecular size (collision radius) and diffusivity (inversely proportional to molecular weight). Also, these relationships are not always linear.Things would differ for non-well mixed systems. So, you should state all the assumptions clearly. $\endgroup$ – WYSIWYG Aug 24 '15 at 5:55
  • $\begingroup$ the problem is, i am a computer science student, my research field is bioinfo, i do not understand so many factors, any case that is valid for most compounds reacting would do me a lot of help. I don't know that, which is why i asked the question $\endgroup$ – girl101 Aug 24 '15 at 6:42

The relative rates are going to depend not on molecular weight so much but the structure of the molecule.

The specific reaction is also very important.

Nucleophilic substitution reactions (some of the first reactions most students learn about in an organic chemistry course).

Let us compare 1-chlorobutane and 2-chloro-2-methylpropane, both $\ce{C4H9Cl}$. However, the 1-chlorobutane is a primary alkyl halide (not very bulky) and 2-chloro-2-methylpropane is a tertiary alkyl halide (bulky).

The bulkier compound reacts faster in a solvolysis reaction:

$$\ce{(CH3)3CCl ->[\ce{H2O}] (CH3)3COH}\ \ \mathrm{fast}$$ $$\ce{CH3CH2CH2CH2Cl ->[\ce{H2O}] CH3CH2CH2CH2OH}\ \ \mathrm{slow\approx no\ reaction}$$

The reason that the tertiary halide reacts faster in this reaction is that its mechanism involves the formation of a carbocation intermediate and the extra carbon groups at the tertiary center provide more inductive stabilization to that carbocaion than the single alkyl group attached to a primary position. In general, in reactions that generate carbocation intermediates, tertiary substrates will react faster.

However, in many reactions bulkier molecules have steric interactions that negatively effect the rate. In a nucleophilic displacement with $\ce{NaI}$ in acetone, 1-chlorobutane will react faster.

$$\ce{(CH3)2CCl ->[\ce{NaI}][\mathrm{acetone}](CH3)3CI}\ \ \mathrm{slow\approx no\ reaction}$$ $$\ce{CH3CH2CH2CH2Cl ->[\ce{NaI}][\mathrm{acetone}]CH3CH2CH2CH2I}\ \ \mathrm{fast}$$

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  • $\begingroup$ bulkier as in with higher weight? $\endgroup$ – girl101 Aug 13 '15 at 11:43
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    $\begingroup$ Usually in organic chemistry I have seen the interpretation bulky = takes up more space and not bulky = heavier. $\endgroup$ – Ben Norris Aug 13 '15 at 20:46
  • $\begingroup$ is there any example of two compound say A and B such that A is more bulkier than B but B has a higher molecular weight than A $\endgroup$ – girl101 Aug 14 '15 at 3:57
  • $\begingroup$ Sure, I consider anything connected to a tert-butyl group to be bulky in the steric sense, for example $\ce{(CH3)3COH}$, where as a functional group at the end of a linear alkane is not (but could have a higher molar mass): $\ce{CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2OH}$. $\endgroup$ – Ben Norris Aug 14 '15 at 10:46
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    $\begingroup$ done! lets see what response i get $\endgroup$ – girl101 Aug 14 '15 at 15:00

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