Ethyl Group
The ethyl group is a modifier of chemical behavior. The ethyl group has the chemical formula CH3CH2--which is often written as C2H5. It is frequently abbreviated to Et--.
The ethyl group is a modified alkane, it is an ethane molecule with a hydrogen atom removed. The ethyl group is an example of an alkyl group, or an alkyl radical.
One of the effects of the ethyl group is that as the molecular weight of the entire molecule increases, the characteristics of the molecule change. The ethyl group has a greater molecular weight than the smallest alkyl radical which is the methyl group. If we consider the alcohol containing each of these two groups we can see several important differences. The two alcohols are methanol (CH3OH) and ethanol (C2H5OH). Ethanol has a boiling point of 172°F (78°C) whereas methanol will boil at 148°F (64.5°C). The increase in boiling point is not due totally to the increase in molecular weight, with this larger alkyl group there is an increased tendency in the alcohol to form hydrogen bonds. As the alkyl group increases in size the alkane properties of the alcohol become more marked. Some reactions that ethanol can undergo are impossible for methanol. For example reduction (dehydrogenation) of ethanol gives the alkene ethene. Methanol cannot be reduced to the corresponding alkene, because there is only the single carbon atom. For humans methanol is poisonous in small quantities whereas ethanol at low concentrations is harmless, although it can be poisonous at higher concentrations.
The free radical of the ethyl group is known to exist as an entity in its own right. This is as an intermediary a number of reactions. The ethyl group is found for example during the thermal degradation of alkanes as is encountered during various petrochemical processes.
When an ethyl group is added to a compound the process is called ethylation. If an aliphatic (straight chain) molecule is being ethylated it occurs by the substitution of the hydrogen atom found in a hydroxyl or amine group. In aromatic compounds (ring molecules) the substitution may be of one of the hydrogens on the ring structure itself. This substitution can be carried out by the Friedel Crafts reaction, which in the case of benzene (C6H6) is represented as:
C6H6 + CH3CH2Cl C6H5CH2CH 3 + HCl
Ethylbenzene is produced, and the ethylation is facilitated by the addition of a small amount of anhydrous aluminum chloride.
Functional groups attached to a benzene ring affects the reactivity of the ring and determines to a large extent where the substitution will take place. A group that makes the ring more reactive than benzene is called an activating group, whereas a deactivating group makes the ring less reactive. The ethyl group is considered to be weakly activating in the case of electrophilic aromatic substitution reactions. For example, if equimolar amounts of benzene and ethylbenzene are treated with a small amount of nitric acid (a nitrating reagent) about 25 times more nitroethylbenzene is produced than nitrobenzene. The added ethyl group makes ethylbenzene about 25 times more reactive to nitration than benzene. Nitration occurs almost exclusively at the ortho- and para-ring positions in ethylbenzene. Very little of the meta-nitroethylbenzene isomer is produced.
There are several important compounds that contain an ethyl group as part of the molecule. Diethyl ether, CH3CH2OCH2 CH3, was used for many years as a surgical anesthetic agent until being replaced by safer nonflammable alternatives. Ethyl formate, HCOOCH2CH3, is used to flavor lemonade. Ethyl chloride, CH3CH2Cl, is a refrigerant and is sometimes applied to skin as a topical anesthetic. Ethylbenzene is a raw material in the commercial manufacturing of styrene.
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