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Nomenclature of Hydrocarbons

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Nomenclature of Hydrocarbons - Lesson Summary

The system of assigning a name to a compound is known as nomenclature. There are two systems for naming organic compounds

  • Common or trivial system 
  • IUPAC system

The trivial names are given on the basis of the source and certain properties of organic compounds.
Ex: Citric acid is named, as it is found in citrus fruits.

In the year 1947 the IUPAC that is the International Union of Pure and Applied Chemistry system of naming compounds was first developed.

The IUPAC system is a systematic nomenclature in which the name of a compound correlates to its molecular structure.

The IUPAC nomenclature system is a set of logical rules devised and used to write a unique name for every distinct compound. According to the IUPAC system of nomenclature, the name of an organic compound consists of a root word, a suffix and a prefix.

Root Word:
The root word indicates the number of carbon atoms in the basic skeleton.

  Number of carbon atoms   Root word    C    Meth    C-C    Eth    C-C-C    Prop    C-C-C-C    But    C-C-C-C-C    Pent    C-C-C-C-C-C    Hex    C-C-C-C-C-C-C    Hept    C-C-C-C-C-C-C-C    Oct    C-C-C-C-C-C-C-C-C    Non    C-C-C-C-C-C-C-C-C-C    Dec

Example: C-C-C-C-C
Root word in the above system is ‘Pent’ (as it contains five carbon atoms).

A suffix designate the  functional groups that may be present in the compound. The suffix is again divided into primary and secondary.

Primary suffix:
Primary suffix  indicates the degree of saturation or unsaturation in the basic skeleton and is added immediately after the root word.
Primary suffix + Root word → Saturated or unsaturated carbon chain

Nomenclature of Alkanes:
For saturated hydrocarbons, the primary suffix “ane” should be added.
Example: The IUPAC name of a molecule which contains single bond between carbon atoms.
CH3-CH: Eth + ane : Ethane
CH3-CH2-CH: Prop + ane : Propane 

Nomenclature of Alkenes:
Hydrocarbons containing double bonds are known as alkenes.  For such hydrocarbons, the primary suffix “ene” should be added to the root word. 
Example: The IUPAC name of a molecule which contains double bond between carbon atoms.
CH2=CH2: Eth + ene:  Ethene
CH3-CH=CH2: Prop + ene:  Propene
In writting nomeclature of alkenes according to IUPAC, it is important to mention the position of double for the molecules which contain more than three carbon atoms.
Root word: But
Prefix: 1-ene
Root word + prefix: 1-Butene

Root word: But 
Prefix: 2-ene
Root word + prefix: 2-Butene

Nomenclature of Alkynes:
Hydrocarbons that contain a triple bond between carbon atoms are known as alkynes and for naming such hydrocarbons the primary suffix “yne “should be added.
Example: The IUPAC name of a molecule which contains triple bond between carbon atoms.
CH≡CH: Eth + yne:  Ethyne
CH3-C≡CH: Prop + yne:  Propyne
In writting nomeclature of alkynes according to IUPAC, it is important to mention the position of triple bond for the molecules which contain more than three carbon atoms.
Root word: Pent
Prefix: 1-yne
Root word + prefix: 1-Pentyne

Root word: Pent 
Prefix: 2-yne
Root word + prefix: 2-Pentyne

Secondary Suffix:
A secondary suffix indicates the functional group present in the carbon compound. Functional groups are defined as  specific atoms, group of atoms or ions which are part of a larger hydrocarbon chain and impart characteristic properties to the compounds.

Nomenclature of a molecules with functinal group:

    Organic Compound     Functional Group     Secondary Suffix to be used    Alcohols   -OH   -ol    Aldehydes   -CHO   -al    Ketones   >CO   -one   Carboxylic acid   -COOH   -oic aid   Acid amides   -CONH 2   -amide   Acid chlorides   -COCl   -oyl chloride   Esters   -COOR   -alkyl...oate   Cyanides   -CN   -nitrile   Thioalcohols   -SH   -thiol   Amines   -NH 2   -amine

A molecule of ethyl alcohol contains two carbon atoms, so the root word should be “eth”.
It is saturated so the primary suffix should be “ane” but as there is a functional group (alcohol) "–OH" in the molecule, remove the “e” from the name of the molecule and add the secondary suffix “ol”. 
Therefore, the IUPAC name of ethyl alcohol is “ethanol”.

CH3-CH2-OH : Eth + an+ol : Ethanol

The IUPAC  name of the propanaldehyde molecule can be written as Propanal, 
Root word:Prop
Primary suffix: an
Secondary suffix: al 
Root word + Primary suffix + Secondary suffix: Propanal

The IUPAC name of acetone can be written as propanone. 
Root word:Prop 
Primary suffix: an 
Secondary suffix: one
Root word + Primary suffix + Secondary suffix: Propanone

And IUPAC name of acetic acid can be written as ethanoic acid
Root word:Eth 
Primary suffix: an 
Secondary suffix: oic acid
Root word + Primary suffix + Secondary suffix: Ethanoic acid

The parts of the name that precede the root word are called prefixes. For example, in the compound, cyclobutane, “cyclo” is the prefix that indicates the alicyclic nature of the compounds.

A primary prefix is used to differentiate acyclic and cyclic compounds. But the rules for using these are slightly different.

Ex: In cyclic compounds, the prefix cyclo is added before the word root.

Functional groups with halogen as the hetero atom are,

Functional group Formula Prefix to be used Flourine -F Flouro Chlorine -Cl Chloro Bromine -Br Bromo Iodine -I Iodo

Nomenclature of molecule with halogen as functinal group:
IUPAC nomenclature of molecule of ethyl chloride.
Root word: Eth
Primary suffix: ane
Prefix: Chloro
Prefix + Root word + primary suffix: Chloro ethane

In case molecules with more than three carbon atoms, it is important to specify the position of halogen.
Root word: Prop
Primary suffix: ane
Prefix:: 1-Chloro
Prefix + Root word + primary suffix: 1-Chloro propane

Root word: Prop
Primary suffix: ane
Prefix: 2-Chloro
Prefix + Root word + primary suffix: 2-Chloro propane


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