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My chemistry textbook says

If detergents are highly branched then they can't be easily degraded by bacteria.So,nowadays the branching of hydrocarbon chain is controlled and kept to a minimum.

Upon searching this I got the answer that "side chains stop bacteria from attacking and breaking the chains". I really don't understand how does this work.Why would having side chains prevent bacteria from attacking ? Won't they provide more sites for attack of bacteria ?

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  • $\begingroup$ I don't know the appropriate Stack topics available for this question.If it should be posted on a different website please notify me via comment. $\endgroup$
    – Sristy
    Commented May 19, 2020 at 20:19
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    $\begingroup$ Hm, it´s not "bacteria", it´s enzymes. All modern standard detergents are completely linear, so is soap made out of natural oils. The enzymes (quite a zoo of them) afaik have trouble reaching a quarternary carbon atom. $\endgroup$
    – Karl
    Commented May 19, 2020 at 21:17
  • $\begingroup$ Does this have anything to do with steric hindrance? If 'breaking chains' is classified as reducing the total length of a compound by making a scission in the detergent backbone, then having too many branched groups may provide a spacial limitation for the enzyme to get to where it needs to. $\endgroup$
    – samp
    Commented May 20, 2020 at 1:23

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Here is a study1 showing this is the case, that generally speaking, branched hydrocarbons are more difficult to degrade, especially those exhibiting anteiso-terminal branching (branching on the third carbon). Both the study and a book on oil spill forensics2 hypothesize that steric hindrance is the mechanism responsible. They state, respectively:

If alkyl branches are located near the terminus, it seems likely that decreased biodegradability could be the result of steric inhibition of terminal oxidizing enzymes

Complex branching hinders both the initial oxidation and the subsequent lipid catabolism apparently because tertiary and quarternary carbon atoms interfere by steric hindrance with the oxidation enzymes.

However, this is not always the case as Boethling3 explains when discussing design objectives for molecule biodegradability:

Similarly there is a commonly held view that any branching, for example, even a single methyl group on an otherwise linear alkyl group, is to be avoided. This is a gross oversimplification. All that can be said with any confidence is that quaternary carbon is usually to be avoided, as is extensive methyl chain branching, which has no strict definition.

1 Schaeffer, T. L., Cantwell, S. G., Brown, J. L., Watt, D. S., & Fall, R. R. (1979). Microbial Growth on Hydrocarbons: Terminal Branching Inhibits Biodegradation. Applied and Environmental Microbiology, 38(4), 742-746. Retrieved from https://aem.asm.org/

2 Wang, Z., & Stout, S. (2006). Oil Spill Environmental Forensics: Fingerprinting and Source Identification (1st ed.). Burlington, MA: Academic Press.

3 Boethling, R. S., Sommer, E., & DiFiore, D. (2007). Designing Small Molecules for Biodegradability. Chemical Reviews, 107(6), 2207–2227. https://doi.org/10.1021/cr050952t

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