The Names of Organic Compounds

8/12/2019 The Names of Organic Compounds

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THE NAMES OF ORGANIC COMPOUNDS

Cracking the code

A modern organic name is simply a code. Each part of the name gives you some useful information aboutthe compound.

For example, to understand the name 2-methylpropan-1-olyou need to take the name to pieces.

Thepropin the middle tells you how many carbon atoms there are in the longest chain (in this

case, !. The anwhich follows the "prop" tells you that there aren#t any carbon$carbon double bonds.

The other two parts of the name tell you about interesting things which are happening on the first

and second carbon atom in the chain. Any name you are likely to come across can be broken up inthis same way.

Counting the carbon ato!%ou will need to remember the codes for the number of carbon atoms in a chain up to & carbons. There isno easy way around this $ you have got to learn them. 'f you don#t do this properly, you won#t be able toname anything

code no o" carbon!

meth )

eth *

prop

but +

pent

hex &

T#$e! o" carbon%carbon bond!

-hether or not the compound contains a carbon$carbon double bond is shown by the two lettersimmediately after the code for the chain length.

code ean!

an only carbon$carbon single bonds

en contains a carbon$carbon double bond

For example, butanemeans four carbons in a chain with no double bond.Propenemeans three carbons in a chain with a double bond between two of the carbons.

A&k#& grou$!ompounds like methane, /+, and ethane, //, are members of a family of compounds calledalkanes. 'f you remove a hydrogen atom from one of these you get an alkyl group.

For example0 A methylgroup is /.

An ethylgroup is //*.

These groups must, of course, always be attached to something else.

T#$e! o" co$ound!

The a&kane!

Example 1: -rite the structural formula for '%eth#&$entane.1tart decoding the name from the bit that counts the number of carbon atoms in the longest chain $pentcounts carbons.

Are there any carbon$carbon double bonds2 3o $ antells you there aren#t any.3ow draw this carbon skeleton0


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4ut a methyl group on the number * carbon atom0

5oes it matter which end you start counting from2 3o $ if you counted from the other end, you would drawthe next structure. That#s exactly the same as the first one, except that it has been flipped over.

Finally, all you have to do is to put in the correct number of hydrogen atoms on each carbon so that each

carbon is forming four bonds.

If you had to name this yourself: ount the longest chain of carbons that you can find. 5on#t assume that you have necessarily

drawn that chain hori6ontally. carbons meanspent. Are there any carbon$carbon double bonds2 3o $ thereforepentane.

There#s a methyl group on the number * carbon $ therefore 2-methylpentane. -hy the number *

as opposed to the number + carbon2 'n other words, why do we choose to number from thisparticular end2 The convention is that you number from the end which produces the lowestnumbers in the name $ hence *$ rather than +$.

Example 2: -rite the structural formula for '()%dieth#&butane.1tart with the carbon backbone. There are + carbons in the longest chain (but! with no carbon$carbondouble bonds (an!.

This time there are two methyl groups (di! on the number * and number carbon atoms.

ompleting the formula by filling in the hydrogen atoms gives0

Note* 5oes it matter whether you draw the two methyl groups one up and one down, or both up, or bothdown2 3ot in the least 'f you aren#t sure about drawing organic molecules, follow this link before you goon. 7se the 8A9 button on your browser to return to this page.

Example 3: -rite the structural formula for '('%dieth#&butane.

This is exactly like the last example, except that both methyl groups are on the same carbon atom. 3oticethat the name shows this by using 2,2-as well as di. The structure is worked out as before0

Example 4: -rite the structural formula for )%eth#&%'%eth#&he+ane.hexanshows a & carbon chain with no carbon$carbon double bonds.

This time there are two different alkyl groups attached $ an ethyl group on the number carbon atom anda methyl group on number *.

Filling in the hydrogen atoms gives0
http://www.chemguide.co.uk/basicorg/conventions/draw.html#tophttp://www.chemguide.co.uk/basicorg/conventions/draw.html#top


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Note* :nce again it doesn#t matter whether the ethyl and methyl groups point up or down. %ou might alsohave chosen to start numbering from the right$hand end of the chain. These would all be perfectly validstructures. All you would have done is to rotate the whole molecule in space, or rotate it around particularbonds.'f you aren#t sure about this, then you must read about drawing organic moleculesbefore you go on.

If you had to name this yourself:

/ow do you know what order to write the different alkyl groups at the beginning of the name2 The

convention is that you write them in alphabetical order $ hence ethyl comes before methyl which in turncomes before propyl.

The c#c&oa&kane!

'n a cycloalkane the carbon atoms are ;oined up in a ring $ hence cyclo.

Example: -rite the structural formula for c#c&ohe+ane.hexanshows & carbons with no carbon$carbon double bonds. cycloshows that they are in a ring.5rawing the ring and putting in the correct number of hydrogens to satisfy the bonding re


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The structure is therefore0

Example 3: -rite the structural formula for ethane%,('%dio&.This is a two carbon chain with no double bond. The diolshows 2$:/ groups, one on each carbon atom.

Note* There#s no particular significance in the fact that this formula has the carbon chain drawn vertically.'f you draw it hori6ontally, unless you stretch the carbon$carbon bond a lot, the $:/ groups look verys

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The Names of Organic Compounds