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Difference Between Prefixes n-, iso-, sec-, neo-, tert- in Organic Chemistry

 n-iso-neo-sec- or s-tert- or t-
Stands fornormalisomerGreek for newsecondarytertiary
StructureThe carbon chain is continuous without any branching; therefore, it represents a straight-chain alkane.

Iso- prefix indicates branching of the straight chain alkane. 

 

The prefix iso- is used when the second carbon of the alkane branch carries one methyl group. So, the terminal isopropyl substituent (CH3)2-CH- refers to the iso- prefix.

Neo-prefix represents a branched-chain alkane.

 

The prefix neo- is used when the branched chain alkane's second carbon carries two methyl groups. The terminal tert-butyl group (CH3)3-C- refers to the neo substituent. So, a neo-prefix mandatorily requires five carbon atoms or more.  

 

(There is no neobutane. The only isomers of butane are n-butane and isobutane.) 

The prefix sec- is used when a functional group is bonded to a secondary carbon of an alkane chain. A secondary carbon has two carbon neighbors.

 

The sec- prefix is commonly used for four carbon alkyl chains—for example, sec-butyl. At five and above carbon alkyl chains, there is more than one possibility of a functional group being attached to a secondary carbon. Hence, the usage is very limited. 

The prefix tert- is used when a functional group is attached to a tertiary carbon of an alkane chain. 

A tertiary carbon has three carbon neighbours. So, tert-carbon attachments start with four carbon alkyl groups.

Note : Note that the prefix iso and neo are used in the common system to distinguish straight-chain alkanes (n-) from branched-chain alkanes of the same molecular formula.The prefixes sec- and tert- are commonly used to indicate the state of a carbon atom in an alkyl molecule to which a functional group is attached. For example, a primary (10) carbon has one carbon neighbor, secondary (2o) - two, tertiary (3o) -three, and quaternary (4o) - four.
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   Structural Isomerism in Organic Chemistry

Structural isomerism refers to the variation in structures between two or more molecules that have the same molecular formula. The way atoms connect within molecules can even translate into different physical and chemical characteristics. 

In this chapter, we provide an in-depth discussion on how to identify structural isomerism in molecules, covering six major subtypes. 

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Functional Groups (if any)

Any functional group, if present, is at the carbon end.

 

In such instances, the carbon carrying the functional group will get the highest priority (or the lowest number).

If present, the functional group, or any substituent always preserves the terminal (CH3)2-CH- (propyl) unit.

 

For example, the functional group in iso-propane is at the C-2 carbon to form isopropyl substituent. Similarly, for iso-butyl, the functional group is at C-3 and C-4 for iso-pentyl.

 

In every case, the numbering prioritizes the functional group over the branching.

The neo- prefix is commonly used for neo-pentane or neo-hexane. 

 

And the associated functional groups are neo-pentyl or neo-hexyl derivatives.

 

 

The functional group is present at the secondary carbon atom of a butane.The functional group is bonded to a tertiary carbon.
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Usage in IUPACThe prefix n - is added to common names and skipped while writing the IUPAC nomenclature.The i- of the iso is considered as part of the alkyl group name while assigning alphabetic priority in IUPAC nomenclature. The n- of neo- is prioritized while alphabetizing in IUPAC nomenclature.The prefix sec- is ignored while alphabetizing in IUPAC nomenclature.The prefix tert- is ignored while alphabetizing in IUPAC nomenclature.
 Thus, the isobutyl and neobutyl will be alphabetized starting with i- and n-, but n-butyl, tert-butyl, and sec-butyl are alphabetized with b. Side note - The number prefixes di, tri, tetra, etc. are ignored in alphabetizing.
Examples

Common name- n-Octane

IUPAC name- octane

Common name-Isopropyl benzene, Isopropyl alcohol. 

Corresponding IUPAC names-(propan-2-yl)benzene,  propan-2-ol

Common name- Neopentane, Neohexane. 

Corresponding IUPAC names-  2,2-dimethylpropane, 2,2-Dimethylbutane

 

Common name- sec-butylbromide, sec-butanol. 

Corresponding IUPAC names- 2-bromobutane, 2-butanol.

Common name- tert-butanol, tert-butyl benzene.

Corresponding IUPAC name- 2-methyl-2-propanol, 1,1-dimethylethylbenzene.

Summarizing the differences in one picture-

Related Quiz - Quiz on different prefixes n-, iso-, neo-, sec-, tert- used in organic chemistry

 

Related Reading 

Structural Isomerism (Part of Introductory Organic Chemistry Course)

Functional Groups (Part of Introductory Organic Chemistry Course)

 

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What are functional Groups? How are they important? (Listen to the Podcast) 

Structural Isomerism

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