Functional Groups III Functional Groups III

Halogenoalkanes Organic compounds with one or more halogens in place of hydrogen are called halogenoalkanes or alkyl halides. These compounds are named as alkanes with the halogen as a substituent. The group names for the halogens are fluoro-, chloro-, bromo-, and iodo-. Example: 1 - iodopropane

Name the following alkyl halides: Halogenoalkanes Name the following alkyl halides: CH3CH2CH2CH2Br

Physical Properties of Halogenoalkanes Halogenoalkanes have relatively low boiling points. The only methyl halide which is a liquid at room temperture is iodomethane; chloroethane is a gas The boiling points reflect the intermolecular forces of halogenalkanes; van der Waals forces. As the carbon chain increases in size the boiling point rises due to increased intermolecular forces. There is also an increase in boiling point as you go from a chloride to a bromide to a iodide (for a given number of carbon atoms) is also because of the increase in number of electrons leading to larger van der Waals forces. There are a lot more electrons in iodomethane than there are in chloromethane.

Physical Properties of Halogenoalkanes

Physical Properties of Halogenoalkanes The carbon-halogen bonds are polar, because the electron pair is pulled closer to the halogen atom then the carbon. This is because the halogens are more electronegative then carbon. This means that in addition to the dispersion forces (van der waals) there will be forces due to the attraction between the permanent dipoles. The size of the dipole-dipole attractions will fall as the bonds get less polar (as you go from chloride to bromide to iodide, for example(. Nevertheless, the boiling points rise! This shows that the effect of the permanent dipole-dipole attractions is much less important than that of the temporary dipoles which causes the dispersion forces.

Physical Properties of Halogenoalkanes The location of the halogen on the carbon chain also effect the boiling point. The temporary dipoles are greatest for the longest molecules. The attractions are also stronger if the molecules can lie closely together.

Physical Properties of Halogenoalkane Halogenoalkanes are only slightly soluble in water. It is difficult for the dispersion and dipole-dipole forces to overcome the hydrogen bonding of water molecules. Halogenoalkanes tend to dissolve in organic solvents which have similar intermolecular forces.

Amines Amines can be considered derivates of ammonia in which one ore more hydrogen have been replaced by alkyl groups. Primary amines have one alkyl group bonded to the nitrogen, secondary amines have two groups on the nitrogen, tertiary amines have three groups and quaternary ammonium salts have four. There are several different ways to name amines that are accepted by IUPAC.

Amines Most amines are prefixed by the word amino -, with the location of the NH2 – group being indicated: for example, 2 – aminopentane and 1,6 – diaminohexane. It is also correct to call them by the longest alkane with the suffix – amine: for example, pentan-2-amine. However, when the number of carbon atoms is small (one, two or three), the old names of methylamine, ethylamine and propylamine tend to be used rather than aminomethane, aminoethane and aminopropane. IUPAC accepts 1 – butylamine, 1 – butanamine and 1 – aminobutance for CH3CH2CH2CH2NH2

1 – butylamine, 1 – butanamine and 1 – aminobutance

Examples include N – methylethanamine and N – ethylpropanamine. Amines In the case of secondary amines the main name of the amine is taken from the longest carbon chain attached to the nitrogen atom. The other chain is prefixed as an alkyl group, with the location prefix given as an italic N. Examples include N – methylethanamine and N – ethylpropanamine. For tertiary amines there are two prefixes with an italic N: for example, CH3CH2N(CH3)2 is N, N – dimethyethanamine.

N,N - dimethylethanamine

Physical Properties of Amines Amines have relatively high boiling points because they can form hydrogen bonds with each other as well as van der Waals dispersion forces and dipole-dipole interactions. The longer the carbon chain the greater the van der Waals dispersion forces and therefore the higher boiling point. Primary amines have higher boiling points than secondary amines due to the nitrogen being in the middle of the secondary amine; causes a lower dipole-dipole attraction. Tertiary amines have much lower boiling points due to the lack of hydrogen atoms on the nitrogen and therefore absence of hydrogen bonding with themselves.

Physical Properties of Amines Smaller amines are very soluble in water; they are able to form hydrogen bonds with water (even tertiary amines). The solubility decreases as the carbon chain increases as would be expected.

Propanamide: CH3CH2CONH2 Amides Amides are derived from carboxylic acids. A carboxylic acid contains the – COOH group, and in a amide the – OH part of that part is replaced by an – NH2 group. So amides contain the – CONH2 group. The name of amides are derived from the acid by replacing “- oic acid” ending with “amide”. Methanamide: HCONH2 Ethanamide: CH3CONH2 Propanamide: CH3CH2CONH2

Amides If the chain was branched, the carbon in the –CONH2 group counts as the number 1 carbon atom.

Physical Properties of Amides The boiling points of the amides are high for the size of the molecules because they can form hydrogen bonds. The hydrogen atoms in the –NH2 group are sufficiently positive to form a hydrogen bond with a lone pair on the oxygen atom of another molecule. Small amides are also soluble in water do to their ability to form hydrogen bonds with water.

Nitriles Nitries (R – CN) used to be called cyanides, so that C2H2CN was known as ethyl cyanide. The IUPAC way of naming nitriles is to consider the acid from which they are derived, as the – COOH group has been replaced by a – CN group. The suffix – nitrile is added to the hydrocarbon forming the basis of the acid. For example, C2H5CN now becomes propanenitrile, and ethanenitrile has the formula CH3CN

propanenitrile ethanenitrile