How do I name this (1 Viewer)

Vaibhav123456

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Similar to a branched alkane with alkyl groups attached (e.g. a -CH3 branch is called methyl as it has one less hydrogen than methane because it is also bonded to a carbon atom). Similarly since the cyclopentane is bonded to another carbon it has one less hydrogen and by replacing 'ane' with 'yl' it becomes a cyclopentyl group. If im wrong someone else can correct me, because im pretty sure this isnt required in the HSC syllabus.
 

CM_Tutor

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The IUPAC name for C5H9-C(=O)-CH3 is indeed cyclopentylethanone.

The carbonyl group is the principal functional group, and it is on a two carbon chain. It can't be an aldehyde (as the carbonyl has no adjacent H atom), so it is a ketone, hence the unusual construction "ethanone". The cyclopentyl ring is then treated as a substituent (as indicated above, just as methyl is a substituent to methylpropane, for example). Also as indicated above, removing one H atom from an alkane / cycloalkane produces an alkyl / cycloalkyl radical.

Since the PFG is assigned the lowest possible locant, it must here be on C#1. The locant for the cyclopentyl group is thus also 1, leading to a name of 1-cyclopentylethanone, but the 1 is not needed as there is no other position on the ethanone parent chain where it could be located and the product still be a ketone.

Using the same logic, if the cyclopentyl group (C5H9) were on the other carbon, the structure would be C5H9-CH2-CH(=O), an aldehyde, and so named 2-cyclopentylethanal, or simply cyclopentylethanal as again the locant is redundant.

Other commonly used names for the first compound would be acetylcyclopentane (as acetyl refers to CH3-C(=O) as a substituent). Acetylmethane would then be an alternative approach to naming propanone (aka acetone), and acetylacetone and diacetylacetone are common names for 2,4-pentanedione and 2,4,6-heptanetrione. Ketones are sometimes also named using the two substituents to the carbonyl, so that butanone becomes methyl ethyl ketone, and by this approach, cyclopentylethanone might be called cyclopentyl methyl ketone.

As for "included in the syllabus", I don't think it would be a reasonable / fair question for an HSC exam or a trial, but schools sometimes ask questions that are borderline in terms of syllabus relevance / reasonableness.

By the way, this would make C5H9-C(=O)-C5H9 be named as dicyclopentylmethanone... so you can have a ketone with a single carbon in its main chain!
 

Jojofelyx

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Similar to a branched alkane with alkyl groups attached (e.g. a -CH3 branch is called methyl as it has one less hydrogen than methane because it is also bonded to a carbon atom). Similarly since the cyclopentane is bonded to another carbon it has one less hydrogen and by replacing 'ane' with 'yl' it becomes a cyclopentyl group. If im wrong someone else can correct me, because im pretty sure this isnt required in the HSC syllabus.
Thank you, another question. Im pretty sure this should be called "propyl pentanoate". But could it be called "butyl butanoate". The -OOC- is confusing me here, which direction do I assume it faces?
 

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CM_Tutor

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Thank you, another question. Im pretty sure this should be called "propyl pentanoate". But could it be called "butyl butanoate". The -OOC- is confusing me here, which direction do I assume it faces?
It is propyl pentanoate and not butyl butanoate.

The C=O must be the C that doesn't have any bound H atoms, otherwise it will end up with too many bonds.

That is, CH3CH2CH2OOCCH2CH2CH2CH3 can only be CH3-CH2-CH2-O-(O=)C-CH2-CH2-CH2-CH3.

Butyl butanoate would be CH3CH2CH2CH2OOCCH2CH2CH3.
 

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