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Longest Chain Rule

When two or m...

Question

When two or more substituents are present on the parent chain, such groups are named in their:

A

alphabetical order

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B

position number

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C

functional order

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D

none of the above

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Solution

The correct option is A alphabetical order
When two or more substituents are present on the parent chain, such groups are named in their alphabetical order. For example, correct IUPAC name for $C H_{3} - C H_{2} - C H (C_{2} H_{5}) - C H (C H_{3}) - C O O H$ is 3-ethyl-2-methylpentanoic acid and not 2-methyl-3-ethylpentanoic acid. Thus, ethyl is named before methyl as in alphabetical order, 'e' comes before 'm'.

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Q. When two or more substituents are attached to the parent chain, these are prefixed in anti alphabetic order.

Q. Are halogens considered as functional groups or side chains or substituents?

Q. Assertion :Prefix is written in front of root word. Reason: A primary prefix is like cyclo- for ring compounds and secondary prefixes are substituents or side chains like alkyl groups e.g. methyl, ethyl.

Q. The various steps for writing the IUPAC name of a saturated hydrocarbon is given below.
1) Identify the substituent groups if present any.
2) Numbering of carbon atoms in principal chain in which the substituent groups will have the lowest number.
3) Select the longest chain of carbon atoms.
4) Naming of the root word can be predicted from the number of carbon atoms present in the principal chain.
5) Suffix will be -ane and prefix will be named according to the substituents.
Which of the given arrangements describes the correct order?

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H_{3} C - C H (C H_{3}) - C H_{2} - C H (C H_{3}) - C H_{2} - C H_{3}

is 2,4-dimethylhexane not 3,5-dimethylhexane. Reason: In case the parent chain has two or more substitution numbering must be done in such a way that the sum of the position number of the locants on the parent chain is the lowest possible.

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