Cycloalkanes - Structure and Nomenclature

In this article we will discuss Structure and Nomenclature of Cycloalkanes. 

Cycloalkenes are cyclic saturated hydrocarbons and have general formula of C_nH_2n. They are named in similar way of alkanes with addition of prefix “cyclo”.

Key words: Saturated hydrocarbons, Cycloalkanes, IUPAC nomenclature. 

Common cycloalkanes are shown below in Table 1

Table 1

Now let’s consider substituted cycloalkanes

For example molecule A

It is a five carbon cycloalkane so the name is cyclopentane. Also there is halide substituent on the ring. Therefore correct name of the compound is “1-chloro-cyclopentane”

Let’s consider the situation where two similary substituents are present on the ring. For example molecule B

When there are two or more similar substituents present on the ring. In that case we should use prefixes like “di”, “tri” or “tetra” as shown in Table 2.

Number of similar groups	Prefix
Two	di
Three	tri
Four	tetra
Five	penta
Six	hexa

 Table 2

Therefore correct name of the molecule B is “1,2-dichloro-cyclopentane”

Now consider another example C where two different substituents are present.

Here parent alkane is “cyclohexane”.

How to give numbers to the carbon then?

In this situation write the substituents in alphabetical order and give carbon number 1 to the first substituent. Therefore correct name of the molecule C is “1-bromo-2-chloro-cyclohexane” and not “1-chloro-2-bromo-cyclohexane”

Also consider molecule D

Here parent alkane is “cyclohexane”. Now if we write substituents in alphabetical order “ethyl” will be first and “methyl” will be second. Therefore correct name of the molecule D is “1-ethyl-2-methyl-cyclohexane”.

Similarly in molecule E

If we write substituents in alphabetical order “bromo” will be first and “methyl” will be second. Therefore correct name of the molecule E is “1-bromo-2-methyl-cyclohexane”.

Now consider the case where three substituents are present, for example molecule F.

If we write substituents in alphabetical order “bromo” will be first, chloro will be second and “methyl” will be third. Therefore correct name of the molecule is F “1-bromo-2-chloro-3-methylcyclohexane”.

Now consider very similar situation in molecule G

As seen before if we write the substituents in alphabetical order then the Br will be attached to C-1, Cl will be attached to C-2 and CH3 will be attached to C-6. Here this numbering is not acceptable because CH₃ gets highest number. According to the rule all the substituents should get least possible number. Therefore at this situation Cl attached carbon is considered as C-1, Br attached carbon is considered as C-2 and CH₃ attached carbon is considered as C-3. Therefore correct name of the molecule G is “2-bromo-1-chloro-3-methyl-cyclohexane”. Here even if the numbers are given differently but substituents are written in alphabetical order. 

Finally to summarize this topic

1.    Cycloalkanes are named by addition of prefix “cyclo” along with the name of alkane.

2.    In case of substituted cycloalkanes, the substituents are written in alphabetical order.

3.    If there are similar substituents are present on the ring in that case prefixes like “di” , “tri”or “tetra” are used.

4.    All the substituents should get least possible number.

That's all for this topic. If you have any questions please feel free to ask me. Also suggest me if any changes or additions are required. Thank you..!