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Section 3: Organic

CHAPTER 27: NOMENCLATURE PLUS BRIEF STRUCTURES AND
PHYSICAL PROPERTIES OF SOME ORGANIC COMPOUNDS
NB This chapter uses the following fonts: Tahoma, Symbol and Windings 3.
Without Windings 3, arrows may appear as letters, such as a 'D' or a 'g', depending on type of arrow.

27.1. ALKANES

Straight chain alkanes are named according to the number of carbon atoms:

CH₄:..................... methane......................... 5 carbon:.... pentane

CH₃CH₃:................ ethane........................... 6 carbon:.... hexane

CH₃CH₂CH₃:.......... propane......................... 7 carbon:.... heptane

CH₃CH₂CH₂CH₃:.... butane........................... 8 carbon:.... octane etc.

Akyl groups are named methyl, ethyl etc.

In branched chain alkanes, the longest unbranched chain is found first, and named accordingly. The position of alkyl groups attached to the main chain is indicated by the number of the carbon atom (in the longest chain) to which they are attached. The name of the side group is given after this number and before the name of the main chain:

.......... 4 ... 3.... 2 .. 1
E.g..... CH₃-CH₂-CH-CH₃....................... 2-methylbutane
........................ |
........................ CH₃

......... 1..... 2.. 3... 4.... 5 ... 6
......... CH₃-CH-CH-CH₂-CH₂-CH₃.......... 3-ethyl-2-methylhexane
................ |.... |
............... CH₃ C₂H₅

................. CH₃
................. |
......... CH₃-C-CH₃............................... 2,2-dimethylpropane
................. |
................. CH₃

Note that: i) Names are all joined together in one long word.

............. ii) Hyphens are placed between numbers and words, and commas between numbers.

............ iii) Numbering of the carbon atoms in the longest chain is started at the end closest to a substituent/side chain.

........... iv) When there is more than one substituent/side chain they are named in alphabetical order.

........... v) When there are substituents/side-chains at equal distances from either end of the longest chain, the numbering is started at the end nearest to the first-named (alphabetical) substituent.

Branched alkyl groups are named accordingly:

......... CH₃
.......... \
........... CH-.......................................... prop-1-yl
.......... /
......... CH₃

Physical properties: C₁ to C₄ are colourless gases, C₅ to C₁₆ are colourless liquids, and higher alkanes are solid. Branching reduces melting point and boiling point by reducing the closeness with which the molecules can pack and thus reducing the extent of van der Waals bonding. Alkanes are generally soluble in organic solvents, but not in water.

27.2. ALKENES

The carbon chain is named in the same way as it is in alkanes, but
"-ane" is replaced by "-ene" to indicate the presence of a double bond. The position of the double bond in a chain with more than three carbon atoms is indicated by numbering:

Examples:......... H₂C=CH₂..................... ethene

....................... CH₃-CH=CH₂................ propene

....................... CH₃-CH₂-CH=CH₂......... but-1-ene

............................... CH₃
............................... |
...................... CH₂=CH-C=CH₂............. 2-methylbuta-1,3-diene

Physical properties: The first four alkenes are colourless gases. Alkenes are insoluble in water but soluble in organic solvents.

27.3. ALKYNES

The naming of alkynes is essentially the same as the naming of alkenes except that the suffix "-ene" is replaced by "-yne".

................... HC=CH............................. ethyne

................... CH₃C=CH......................... propyne

Physical properties: Both the above alkynes are colourless gases which are insoluble in water but soluble in organic solvents.

27.4. BENZENE AND ITS DERIVATIVES

The structure of benzene is disccussed in chapter 3.

When there is more than one substituent in a benzene ring, their relative positions are indicated by numbering. The numbering is chosen to give substituents the lowest possible numbers and, when relevant, they are placed in alphabetical order:



Note that carbon-2 used to be known as the ortho- (o-) position, relative to carbon-1 in a disubstituted compound. Similarly carbon-3 was known as the meta (m-) position, and carbon-4 as the para (p-) position.

Sometimes, it is less clumsy to name a benzene derivative "the other way round" in which case the benzene group is called a phenyl group.



Physical properties: Benzene and the derivatives shown here are all volatile liquids which are insoluble in water and miscible with organic solvents.

27.5. HALOGEN COMPOUNDS

Halogen compounds are named systematically as halogen substituted hydrocarbons:

E.g................ CH₃CH₂Cl............. chloroethane, a primary haloalkane

.................... CH₃
..................... \
...................... CH-Br.................. 2-bromopropane, a secondary haloalkane
..................... /
.................... CH₃

.................... CH₃
.................... |
.................... CH₃-C-I................ 2-iodo-2-methylpropane, a tertiary haloalkane
.................... |
.................... CH₃

.................... CH₂=CH-CH₂Cl..... 1-chlorprop-2-ene



Trivial names such as ethylchloride are still used. The terms primary, secondary and tertiary refer to the number of alkyl groups attached to the functional carbon, except in halomethanes, which are primary haloalkanes with only one carbon atom/alkyl group. Compare this with primary, secondary and tertiary alcohols (section 27.6.) and amines (section 27.10.)

Physical properties: Chloromethane, bromomethane, and chloroethane are colourless gases at room temperature. Other halohydrocarbons are colourless liquids.

27.6. HYDROXY COMPOUNDS

Alcohols are named by using a suffix, "-ol", rather than a prefix like that used in the case of haloalkanes. (Systematic nomenclature is not as consistent as it might be.) The suffix replaces the "-e" in the parent alkane's name.

.......... CH₃CH₂OH...................... ethanol, a primary alcohol

.......... CH₃
........... \
............ CHOH............................ propan-2-ol, a secondary alcohol
........... /
.......... CH₃

.......... CH₃
.......... |
.......... CH₃-C-OH....................... 2-methylpropan-2-ol, a tertiary alcohol
.......... |
.......... CH₃



Physical properties: The lower alcohols are colourless liquids with higher boiling points than might be expected. This is a result of hydrogen bonding between molecules (section 22.1.7.ii.).

As well as being miscible with organic solvents, and provided the hydrophobic carbon chain is not too long, alcohols are miscible with water due to hydrogen bonding with water (section 22.1.7.). Phenol is also quite soluble.

2-methylpropan-2-ol and phenol are low melting point solids.

27.7. ETHERS

Part of the ether (-OR) is considered as an alkoxy group attached to the other (longer if there is a difference in chain length) R group:

E.g............ CH₃
.................. \
................... O........................... methoxyethane
.................. /
........... CH₃CH₂

The most common ether, ethoxyethane, is also still known as diethylether, or simply as ether.

Physical properties: Ethers are liquids, but far more volatile than the alcohols with which they are isomeric. This is because there is no possibilty of hydrogen bonding. They are also immiscible with water.

27.8. CARBONYLS

i) Aldeyhdes are named using the usual prefixes to indicate cahin length, and the suffix "-al" to indicate the aldehyde group. Note that aldehyde groups can only be terminal:



Methanal is still regularly called formaldehyde, and ethanal is often called acetaldehyde.

Aromatic aldehydes are named by adding the suffix carbaldehyde to the name of the parent hydrocarbon. Thus C₆H₅CHO should strictly be known as benzenecarbaldehyde, but is usually called benzaldehyde.

ii) Ketones are again named using the usual prefixes. The ketone group is denoted by the suffix "-one", and the position of the group is indicated by numbering of the carbon chain.

E.g...... CH₃................................... CH₃CH₂CH₂
........... \..................................................... \
........... C=O....... propanone......................... C=O....... pentan-2-one
........... /..................................................... /
.......... CH₃................................................ CH₃

Aromatic ketones tend, even now, to be known by their trivial rather than systematic names:



Physical properties: Methanal is a colourless gas, but the other carbonyls are colourless liquids. Methanal has a sharp but musty smell. Ethanal has a sharp smell described as being like fresh apples or tomatoes. The lower carbonyls are soluble in / miscible with water, as well as organic solvents. A 40% solution of methanal in water is the preserving solution "formalin".

27.9. CARBOXYLIC ACIDS AND THEIR DERIVATIVES

27.9.1. Carboxylic acids: The aliphatic carboxylic acids are named after the corresponding alkanes, the suffix "-oic acid" replacing the "-e".



Aromatic acids are named using the name of the hydrocarbon followed by "carboxylic acid" to indicate the presence of the acid group:



Physical properties: The lower carboxylic acids are colourless liquids which are miscible with water. The solubility in water is due to hydrogen bonding between the carboxylate group and water. This is increasingly counteracted as the length of the hydrophobic carbon chain increases. Their boiling points are higher than expected due to association between pairs of molecules (section 22.3.4.)

27.9.2. Carboxylic acid derivatives are named reasonably logically, as can be seen from the following examples. The examples also show the structures of the functional groups:



the smallest acid chloride. It is a colurless liquid which reacts with water (22.3.6.i)



the smallest acid anhydride. It is a colourless liquid which reacts with water (22.3.6.i.).



an ester. If you find it difficult to decide which part of the name refers to which alkyl chain, compare with sodium ethanoate. The simpler esters are colourless volatile liquids with pleasant, often fruity, smells. They are insoluble in water but react with it on prolonged heating (22.3.6.i.)



a salt.



the smallest amide. Methanamide is a high boiling point colourless liquid, the others are white solids. The lower members are soluble in water.

For names of aromatic derivatives see study question 7.

27.10. AMINES

Amines divide into primary, secondary, and tertiary amines, but the terms describe the number of alkyl groups attached to the functional nitrogen. Compare this with the terms as applied to haloalkanes and alcohols (sections 27.5. and 27.6.). With amines, the series is completed by quarternary ammonium salts. All the members of the series are named according to the alkyl groups present:

E.g........... CH₃NH₂............ methylamine, a primary amine

.................CH₃
..................\
...................NH.................. dimethylamine, a secondary amine
................./
................CH₃

................CH₃
.................\
..................CH₃-N.............. trimethylamine, a tertiary amine<
................./
...............CH₃

...............CH₃ ....+
...............|
.......H₃C-N-CH₃....... Cl^-..... tetramethylammonium chloride,
...............|.........................a quarternary ammonium salt.
...............CH₃

...............C₆H₅NH₂............ phenylamine, a primary amine

...............C₆H₅CH₂NH₂...... (phenylmethyl)amine, a primary amine

...............C₆H₅NHCH₃........ N-methylphenylamine, a secondary amine
BUT:
...............CH₃
................\
................CH-NH₂............. 2-aminopropane, a primary amine
.............../
..............CH₃

Physical properties: Methylamine, ethylamine, and dimethylamine are colourless gases, the other lower amines are colourless liquids. Association via hydrogen bonding gives them higher boiling points than might be expected:



They tend to have a fishy smell, and the lower amines are soluble in water, again due to hydrogen bonding.

27.11. NITRILES

The "-e" in the name of the corresponding alkane is replaced by "-onitrile":

E.g........ CH₃-C=N....... ethanonitrile (methyl cyanide)

..............CH₃-N=C....... methanoisonitrile (methylisocyanide)

Aromatic nitriles are named after the parent hydrocarbon plus "carbonitrile"):

E.g........ C₆H₅C=N....... benzenecarbonitrile (benzonitrile)

Physical properties: Nitriles are liquids which are largely immisible with water, but slowly hydrolysed by it. Isonitriles have a foul smell.

27.12. NITRO COMPOUNDS

Are named in a similar way to halocompounds:



Physical properties: Nitro compounds are liquids which are largely immiscible with water, but slowly hydrolysed by it. Nitrobenzene smells of almonds.

Note that nitroalkanes are isomeric with alkyl nitrates (R-O-N=O) which are inorganic esters of nitric(III) acid.

27.13. QUESTIONS

1) Look at the structures of all the main classes of compound listed in this chapter. Describe clearly in words the characteristic part of the structure for each class of compound.

2) Using the information in this chapter, together with information in

chapter 3, draw orbital overlap structures for: methanal, benzenecarbaldehyde, methanoic acid, nitrobenzene.

Note that, when applicable, your diagrams will give you a fuller understanding of delocalisation.

3) Systematic nomenclature is not as consistent as the name implies. Comment.

4) Account for the fact that phenol is less soluble than ethanol in water, yet phenol more readily dissociates into ions?

5) From the examples given in section 27.9.2., describe the simple rules governing the naming of carboxylic acid derivatives. The sentence describing the naming of carboxylic acids (section 27.9.1.) gives the format.

6) Predict the physical properties of sodium ethanoate.

7) Write structures for phenylbenzenecarboxylate, benzenecarboxamide, phenylethanoylchloride, 2-phenylpropanamide, and N-phenylpropanamide.

8) Write structures for propan-2-ol, 2-aminobutane, 2-chloro-2-methylpropane, and 2-amino-2-methylpropane, stating whether the compounds are primary, secondary, or tertiary.

Unless otherwise stated, all materials in this web version of chapter 27 are © 2007 Adrian Faiers MA (Oxon) MCIPR