Carboxylic acids, Acid chlorides and Acid anhydrides

Carboxylic acids

Carboxylic acids are organic acids that have the functional group. Despite having the same functional groups as both a carbonyl and an alcohol, the carboxylic acid shows few similarities to that of aldehydes and ketones or primary alcohols.

Carboxylic acids have the suffix -anoic after there name.For example an acid with 3 carbons in it's chain would be called propanoic acid. Traditional name are often used for the common acids ;

e.g Methanoic acid is commonly known as formic acid

Ethanoic acid is known as acetic acid (major ingrediant of vinegar)

Physical properties

Carboxylic acids are able to hydrogen bond between molecules and also to water, making them very soluble. Although the boiling point of the acids is high compared to hydrocarbons of similar RMM it is not as high as expected as the acids are able to form dimers; that is they form in pairs resembling a six pointed ring. Carboxylic acids have a very unpleasant smell which is in contrast to the sweet, fruity smell of the esters which they are isometric with.

This is picture of Methanoic acid, the smallest carboxyllic acid.

Chemical properties

Carboxylic acids are formed by the oxidation of primary alcohols or aldehydes. They will not oxidize further chemically, but will burn in air to form CO2 and H2O.Most acids are weak compared to mineral acids such hydrochloric acid.The strength of an organic acid depends on the constituents near the crboxyl group. For example if an electron withdrawing group such as a chloride group was attatched to a carbon near the carboxyl group then this would 'pull' electron charge away from the O-H group and hence reduce the hold on the H of the O-H group (it is the ability to lose this proton that makes for the acidity of a solution of the compound). Carboxylic acids can be reduced to primary alcohols by a strong reducing agent such as LiAlH4 in dry ethoxyethane.

Some of the more important reactions of Carboxylic acids:

One of the most important reactions of carboxylic acids is the conversion to an acid halide.

Acid Halides

Acid chlorides are prepared from the corresponding carboxylic acids by thionyl chloride, phosphorus trichloride or phosphorus pentachloride. They have the same structure as carboxylic acids but the O-H of the acid is replaced by a halide.

Acid chlorides are the most reactive of the carboxylic acid derivatives. Acid chlorides are very unstable in moist conditions and give off HCl gas on reaction with water.

Some of the important reactions of acid chlorides:

Acid anhydrides

Organic anhydrides are colourless liquids regarded as being formed from from two molecules of carboxylic acid by the removal of a water molecule.

They are usually prepared by distilling a mixture of the acid chloride and the sodium salt of the carboxylic acid in a dry apparatus and collecting the anhydride as the distillate.

Acid anhydrides are less reactive than acid chlorides and are used to prepare amides and esters, especially of phenols where direct esterification is not possible. Acid anhydrides are hydrolyzed to there parent carboxylic acids with water (as shown below).

Carboxylic acids.pdf Version

Hydrocarbons (Alkanes, Alkenes and Alkynes)

As the name suggest, hydrocarbons are compounds containing carbon and hydrogen only. They are mainly derived from coal, natural gas, petroleum and plants. They may be saturated or unsaturated, they may be aliphatic or aromatic. Saturation is the amount of substituents around carbon, in the case of alkanes each carbon has 4 substituents, the maximum allowed by valency, therefore they arer known as saturated compounds. Alkenes and alkynes have double and triple bonds respectively therefore do not have the maximum 4 valency, these types of compounds are therefore unsaturated compounds.

Hydrocarbons are the basis of many fuels: such as butane for gas cookers etc, Acetylene (ethyne) for weilding equipment, and the mixtures of hydrocarbons for motor fuel.

Naming Hydrocarbons:

1. Find the longest possible continuous chain of carbon atoms in the molecule. The molecule will be named as a derivative of this alkane.

i.e	Chain	Name
1 2 3 4 5 6 7 8 9 10		= meth- = eth- = prop- = but- = pent- = hex- = hept- = oct- = non- = dec-

2. Determine the names of the hydrocarbon groups attached to this chain. These side chains are called alkyl groups. There names are obtained by dropping the ending -ane from the corresponding alkane and replacing it with -yl i.e a side chain with 3 carbons would by propyl-.
3. Number the carbon atoms along the chain. Identify the alkyl groups by the number of the carbon atom that they are attached to.
4. If more than one alkyl group of the same type is attached to the molecule then use the prefix di- (two) tri- (three) tetra- (four) in front of the hydrocarbon name.
5. If several types of alkyl groups are attached, namer them in alphabetical order.
6. If the hydrocarbon contains unsaturation then find the number of the carbon with the double or triple bond on it and prefix this number in fron of the ending -ene (alkenes) and -yne (alkynes)

Alkanes

Alkanes are saturated hydrocarbons. The simplest hydrogen is methane (CH4) which is tetrahedral in structure.

There are also more complicated alkanes, such as branched chain alkanes and cycloalkanes. A common occuring cycloalkane is cycloheaxane:

Alkanes are very unreactive apart from cracking and burning in air with a non-luminous flame.

The only react by substituting radicals for hydrogen atoms:

Free radical substitution of Alkanes

At 20'c and in the dark Chlorine and methane do not react. If the mixture is heated above 200'c or is irradiated by ultra violet light a rapid chain reaction takes place.

This chain reaction takes place in three steps:

1. INITIATION :- the Cl2 undergoes homolytic fission and splits into two Cl' radicals.
2. PROPAGATION :- a radical combines with a stable molecule to give a different radical and molecule.

e.g CH4 + Cl' ------------> CH3' + HCl

A chain reaction is then set up when the radicals are combining with stable molecules. This continues until no more product can be formed i.e CCl₄ or the reaction is terminated.

3. TERMINATION :- the reaction can be terminated in three ways: (all involve the combination of 2 radicals)

i) Cl' + Cl' -----------> Cl₂

ii) CH₃' Cl' --------------> CH₃Cl

iii) CH₃' + CH₃' -----------------> CH₃CH₃

Alkenes

This is ethene. The smallest alkene.There is a sigma and a pi bond between the C=C. This means that the bond is shorter and stronger than a single bond.

Alkenes are unsaturated hydrocarbons containing one or more double bonds. Alkenes are prepared by the dehydration of alcohols or by the dehydrobromination of bromoalkanes.

Alkenes are named by the name of the straight chain alkane with a number representing the position of the double bond. The prefixes cis- and trans- (now Z- and E-) are also used to name the simple geometric isomers (see page on isomerizm).

Alkenes undergo electrophilic addition reactions which follow Markovnikovs rule. Alkenes react with halogens such as Br-Br or hydrogen halides such as H-Br. They also react with bromine water and sulphuric acid. All these reagents are able to react with alkenes because they have an electrophilic part on there molecule i.e H-Br has an electrophilic (electron deficient) H which is attracted to the electron rich area of a double bond. The electrophilic part attatches itself to the carbon with the most hydrogens already attached to it, and then the nucleophilic part of the molecule bonds to the carbocation intermediate.

Alkynes

This is ethyne. The smallest alkyne. The C=C triple bond is shorter and stronger than that of an alkane or an alkene.

Alkynes are unsaturated hydrocarbons with one or more triple bonds. They are formed by the hydrolysis of calcium dicarbide or by the dehydrohalogenation of 1,2-dibromoethane using hot alcoholic potassium hydroxide. Alkynes do not undergo many reactions, but are used industrially in cutting and weilding equipment.

Hydrocarbon (Alkanes, Alkenes, and Alkynes) pdf version