lesson 2: IUPAC naming (Nomenclature) of organic compounds

Lesson 2: IUPAC NOMENCLATURE OF ORGANIC COMPOUNDS

Behavioural objectives: at the end of the lesson, you should be able to:

·         Define organic compounds

·         Define homologous series

·         State the general characteristics of homologous series

·         Identify organic compounds with their functional group

·         Give the correct IUPAC name of organic compounds

·         Write the molecular formula and molecular structure of organic compounds

Content

What is organic chemistry?

Organic chemistry is defined as the branch of science which deals with the study of organic compounds.

What is organic compound?

Organic compounds are compounds that contain majorly hydrogen and carbon atoms. Also we can say organic compounds are hydrocarbons. However, some organic compounds may have some other atoms other from hydrogen and carbon attached to them like the case of alcohol which have a functional of (OH). These atoms or radicals (e.g. nitrogen, sulphur, chlorine, cyanide e.t.c.) are refer to as Hetero atoms.  

What is homologous series?

These are family of organic compounds which follows a regular structural pattern in which each successive member of the family differs in its molecular formula by an increase of – CH­₂. Each successive member of the series is called an homolog. 

What are the characteristics of homologous series?

1.      All member of the series can be prepared by the same general method

2.      All member of the series share the same general molecular formula e.g C_nH_2n+2 for alkanes, C_nH_2n for alkenes. And so on like that where “n” is integer

3.      Each member in a series differs in molecular formula by the addition of –CH₂

4.      Each successive member in a series differs in relative molecular mass by an increase of 14 (C=12, H=1)

5.      Their physical properties changes gradually as the number of carbon atoms increases

6.      All members of the same series have similar chemical properties with gradual variation  

Homologous series	General formula	Functional group
Halo alkanes	R – X( where X is Halogen group)	-X
Alkane	R-H	- (single covalent bond)
Alkenes		=
Alkynes		≡≡
Alkanoic (carboxylic acids)	R-COOH	-          COOH
Alkanol	R-OH	-          OH
Alkanoates (esters)	R-COO-R’	-          COO-R’
Alkanals	R-C = O	-          CHO
Alkanones	R-C=O	-          CO -
Ethers	R –O-R’	-          O -
Nitrile	R-C≡N	-          C≡N
Cyano group	R-CN	-          CN

               What is functional group? Functional groups are special atoms or radicals that identify one homologous series from the other and also determine the chemical properties of the compound.

IUPAC NOMENCLATURE OF ORGANIC COMPOUND

Before we look at this let me explain some things here first. If you look at the examples of homologous series in the table above, each family ends with different suffix line –ane, -oate, yne e.t.c. let’s call that suffix their family name just like you have your family name like Moronfolu family. All the members of that family will bear Moronfolu as their Surname in addition to their personal name. let’s take alkanes family for example and name the first ten members of the family.

Name	Molecular Formula
Methane	CH4
Ethane	C2H6
Propane	C3H8
Butane	C4H10
Pentane	C5H12
Hexane	C6H14
Heptane	C7H16
Octane	C8H18
Nonane	C9H20
Decane	C10H22

If you observe the above table, you will realize that all the members name ends with “ane” while having different prefix corresponding to the number of carbon atoms in the compound. We are going to refer to that as Parent Chain. while naming the compounds using the IUPAC naming method.

Yea before we proceed, what is IUPAC? IUPAC is an acronym meaning International Union OF Pure and Applied Chemistry. Now to name any organic compound using IUPAC methods certain rules must be taken into consideration. The rules are as follow:

1.      Identify the functional group and consider the possible longest chain. e.g.

From the above diagram, you could see that one CH₃ is circled on carbon number three, we will regard to that as a branch or a substituent. The compound is another group of organic compound known as alkyl group, with the general molecular formula C_nH_2n-1. Where n is an integer starting from 1. So for the circled one its name is called methyl. Other members of this alkyl group include:

Ethyl                           C2H5

Propyl                          C3H7

Butyl                           C4H9              e.t.c

Now going back to the main point, if look at the image properly, it was numbered from both end, this is necessary to determine the longest chain in the compound.

Also so far that there the functional group here is alkane, you are free to number or count your carbon from any angle. But the lowest position of the substituent (I mean the methyl attached to its now) will be considered in a situation like this.

Therefore, counting from the right side, we have six carbon chains as the longest chain and that is hexane which is the parent chain of the compound. Besides the methyl is located on carbon number 3 which is the lowest location after counting from both ends.

Considering all these facts, the correct IUPAC name for this compound is

 3-methylhexane. The structure in the above image is called the molecular structure. It can also be written as : CH3-CH2-CH2-CH(C2H5)CH3, and this is called the condensed structure.

Example 2:

Looking at the second example in the image above, I numbered it from one side only. The reason is because on carbon number 2 there is a double covalent bond there and that automatically define the side to number your compound. So far the double bond is a functional group. And that will lead us to another rule later. You can see that CH3 is also circled out here, it’s because after counting from both side the longest carbon chain is six so that methyl will stand out as a substituent. Hence the name of this compound 4-methylhex-2-ene. Considering the parent name of the compound which is an alkene family and we have six carbon atoms here, also the double bond is on number 2, so to indicate the position of the double bond we call it hex-2-ene. Mind you, when writing IUPAC name of organic compounds thee should be an hyphen between a letter and a figure.

RULE NUMBER TWO SAYS: there is no branch on the first and the last carbon.  How do I mean?

  

Looking at the above image, if you count linearly from carbon 2 to 4, it means we will consider carbon number 1 and 5 as a branch. But the rule says there is no branch on the first and last carbon. i.e. whenever you see a structure like this, the carbon that branch at both ends is numbered along the parent chain to determine the possible longest chain. Hence the name of the compound (A) is simply Pentane and that of compound (B) is Hex-2-ene.

Rule number 3: says when there is a combination of two or substituents, the substituents are named alphabetically.  

the longest chain we have in this diagram above is six, but looking at the functional group,it is an alkene. hence, the lowest position of the functional group is considered while numbering. however according to the rule ethyl is named first before methyl, regardless of its position simply because letter "e" comes first before letter "m". therefore, the compound is named 4-ethyl-3-methylhex-1-ene.

another example:

4-ethyl-3methylhex-2-ene.

CH2-CH꞊C(CH3)C(C2H5)CH2-CH3

Rule number four says: When there is combination of two different inorganic substituents, the substituents are named in alphabetical order.

In my definition of organic compounds I told us that we can have other atoms apart from Hydrogen and Carbon. Therefore in the case where we have more than one hetero atoms on a compound, we do name alphabetically. Just like the example in the below.

CH3-C(F)(Br)-C-(CH3)-(I)-CH-(C2H5)-CH2-CH3

Counting the carbon from the left, we have bromine and fluorine on carbon number 2, methyl on carbon number 3, Iodine on carbon number 3, ethyl on carbon number 4. Hence following the rules the name of the compound is given as:

CH3-C(F)(Br)-C-(CH3)-(I)-CH-(C2H5)-CH2-CH3

2-Bromo-2-Fluoro-3-Iodo-4-ethyl-3-methylhexane.  

Rule number 5 says: that when there is combination of organic and inorganic substituents, in organic substituents are named first. The compound can be made more complex when they bear both organic and inorganic substituents at the same time, so to name them inorganic comes first. E.g.

CH3-C(F)(Br)-C-(CH3)-(I)-CH-(C2H5)-CH2-CH3

2-Bromo-2-Fluoro-3-Iodo-4-ethyl-3-methylhexane.

Rule number 6 says that: the least position of the organic substituents supersedes the least position of the inorganic substituents. When I said we are to name the inorganic first but since we can count the carbon chain from both ends, there will be a problem on where to start counting. So to solve that, just consider the lowest position of the organic substituents.

CH3-C(F)(Br)-C-(CH3)-(I)-CH2-CH2-CH3

2-Bromo-2-Fluoro-3-Iodo-3-methylhexane.

Rule number 7 says that: the least position of the functional group supersedes the least position of the substituents: in a situation whereby you have a functional group and two or more different substituents like that of alkene or alkyne, you might be confused of what the name would be. The first thing you should take into consideration is the position of the functional group.  E.g.

CH3-CH(CH3)-CH2꞊CH2-CH2

4-methylpent-2-ene

Practice exercise:

Draw the structure of the following compounds.

1.      5-astato-4,4-difluoro-5-iodo-2-methylpentane

2.      1,1-dibromoethane

3.      1,2-dibromoethane

4.      3-bromo-2-iodo-4-methlyhexan-2-ene

5.      But-1,3-diene

6.      5,5-dibromo-6-iodo-4-ethyl-3-methylhex-1,3-diene

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