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Organic Chemistry 101: Nomenclature

Updated on January 17, 2018

Carbon

As I am sure you know, carbon is the backbone of organic chemistry. The number of carbons in a molecule dictates the name it will be given. The number of bonds between the carbon atoms indicates how strong the molecule is. The shape the taken by the carbon atoms in a molecule can change everything. In short everything in organic chemistry revolves around the carbon atom.

Bonds

There are three different bonds in organic chemistry. The single, double, and triple bonds. The more bonds there are the the stronger the connection is. It takes a lot more energy to break a triple bond than it does to break a single bond. The distance between atoms is also shorter between triple bonds than it is between single bonds. The bonds are stable because of the attraction between he electrons and the nuclei.

Source

Alkanes

What is an alkene? It is a molecule composted entirely of carbon and hydrogen atoms connected entirely by single bonds. The name of the molecule is given based upon the number of carbon atoms.

Naming Alkanes

Number of Carbon Atoms
Prefix
Name
Structure
1
Meth
Methane
CH4
2
Eth
Ethane
CH3CH3
3
Prop
Propane
CH3CH2CH3
4
But
Butane
CH3CH2CH2CH3
5
Pent
Pentane
CH3CH2CH2CH2CH3
6
Hex
Hexane
CH3CH2CH2CH2CH2CH3
7
Hept
Heptane
CH3CH2CH2CH2CH2CH2CH3
8
Oct
Octane
CH3CH2CH2CH2CH2CH2CH2CH3
9
Non
Nonane
CH3CH2CH2CH2CH2CH2CH2CHCH2CH3
10
Dec
Decane
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
Each of these structures is a straight line of carbons with the hydrogens coming off, all atoms connected by single bonds.

Branching Alkanes

Naming branched alkanes is simple. First just count the number of carbon atoms in that parents chain. The parent chain will have the greatest number of carbon atoms. Then figure out the name of the parent chain. (propane, ethane, hexane, and so on and so forth) Now figure out the name of the substituent chain, this will be the chain with the lower number of carbon atoms. When naming the molecule you will use the prefix for the substituent and then add on 'yl'. To name the molecule find the carbon atom where the molecule branches off (5, 3, 7, or 2), then give the name of the substituent, and then the name of the parent chain.

Example: if the two chains branch on the 3rd carbon in the parent chain, and the parent chain is methane, and the substituent chain is an ethane. 3-ehtylmthane. If the branching carbon is on carbon 5, the substituent is propane, and the parent chain is hexane then the molecule is 5-propylhexane.

More Than One Substituents

What if there is more than one substituents in the molecule? Simply find the parent chain and the name the substituents. The substituents will be listed in alphabetical order in the name. The carbon atoms with the substituents will still be numbered.

Example: if there is a methane on the 2nd carbon and an ethane is on the 4th carbon of a heptane chain the name is 4,2-ethylmethylheptane.

If the substituents are identical then you add the prefix 'di', 'trip', and so on and of forth. If there is an ethane on the 4th and the 6th carbons of a decane chain then the name is 4,6-diethyldecane.

A Great Song to Help Remember Naming

Cyclic Alkanes

Naming rings is easy. Simply add 'cyclo' to the alkane base. Remember that substituents are on the lowest number of carbons. This means that you might need to change the direction in which you count in the ring. If you have a ring with six carbons and there is an ethane off the second carbon on the right side then the ring is 2-ethylcyclohexane. The same rules for naming non-cyclic structures applies to the cyclic structures.

Rings as a Substituent

If the number of carbons in the ring are not greater than the number of carbon atoms in the chain the the ring is a substituent.

Example: If there is a five carbon chain with a methane on the second carbon and a cyclic propane on the fifth carbon the name would be 2,5- methylcyclopropane.

Alkenes and Alkynes

An alkene is a group of carbons have have a double bond somewhere between two carbon atoms. Where as alkynes have a triple bond between two carbons. The more bonds between carbon atoms the fewer hydrogen atoms they have attached to them. For example if there is a double bond between the second and third carbon on a four carbon chain then the second and third carbons will only have one hydrogen attached instead of two. Now take that sample chain and replace it with a triple bond and now there are no hydrogens attached to carbons two and three. Why? Because there are not enough electrons to share between the atoms. The attraction with the double or triple bond is stronger than the attraction between the carbon and hydrogen.

Single Bond: CH3CH2CH2CH3 Butane

Double Bond: CH3CHCHCH3 Butene

Triple Bond: CH3CCCH3 Butyne.

For naming the molecule use 'ene' for molecules with double bonds and 'yne' for molecules with triple bonds. All of the other naming rules apply after that.

Does naming organic compounds make more sense?

See results

Functional Groups

Functional Groups

The functional groups are typically at the end of a carbon chain. There addition changes how reactive the molecule is with other molecules. They become more import the further you go into organic chemistry.

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