Download Alcohols I. Preparation and Physical Properties Structure Alcohols

Alcohols I. Preparation and
Physical Properties
Structure
Alcohols are compounds of the general formula ROH, where R is any alkyl
or substituted alkyl group. The group may be primary, secondary, or
tertiary;it may be open-chain or cyclic; it may contain a double bond, a
halogen atom, or an aromatic ring. For example:CH,CH r C CH3OH tenButyl alcohol CM -CH 2OH AIM alcohol O1 Cyclohcxanol>CH 2OH Benzyl
alcohol CH2- < \\ 2 Cl OH Ethylene chlorohydrm(/3-Chloroethyl alcohol)
CH^ -CH—CH,-OH OH OH Glycerol
All alcohols- contain the hydroxyl (- OH) group, which, as the functional
group, determines the properties characteristic of this family. Variations in
structure of the R group may affect the rate at which the alcohol undergoes
certain reactions, and even, in a few cases, may affect the kind of reaction.
Compounds in which the hydroxyl group is attached directly to an aromatic
ring are not alcohols; they are phenols, and differ so markedly from the
alcohols that we shall consider them in a separate chapter.
Classification
We classify a carbon atom as primary, secondary, or tertiary according to
the number of other carbon atoms attached to it (Sec. 3.1 1). An alcohol is
classified according to the kind of carbon that bears the OH group
One reaction, oxidation, which directly involves the hydrogen atoms
attached to the carbon bearing the OH group, takes an entirely different
course for each class of alcohol. Usually, however, alcohols of different
classes differ only in rate or mechanism of reaction, and in a way
consistent with their structures. Certain substituents may affect reactivity in
such a way as to make an alcohol of one class resemble the members of a
different class; benzyl alcohol, for example, though formally a primary
alcohol, often acts like a tertiary alcohol. We shall find that these variations,
too, are consistent with the structures involved.
Nomenclature
Alcohols arc named by three different systems. For the simpler alcohols the
common names, which we have already encountered (Sec. 5.19), are most
often used. These consist simply of the name of the alkyl group followed by
the word alcohol. We should notice that similar names do not always mean
the same classification; for example, isopropyl alcohol is a secondary
alcohol, whereas isobutyl alcohol is a primary alcohol. It is sometimes
convenient to name alcohols by the carbinol system. According to this
system, alcohols are considered to be derived from methyl alcohol,
CH3OH, by the replacement of one or more hydrogen atoms by other
groups.
We simply name the groups attached to the carbon bearing the OH and
then add the suffix -carbinol to include the C OH portion: Finally, there is
the most versatile system, the IUPAC. The rules are:
(1) Select as the parent structure the longest continuous carbon chain that
contains the OH group \ then consider the compound to have been derived
from this structure by replacement of hydrogen by various groups. The
parent structure is known as ethanol, propanol, butanol, etc., depending
upon the number of carbon atoms; each name is derived by replacing the
terminal -e of the corresponding alkane name by -ol.
(2) Indicate by a number the position of the OH group in the parent chain,
generally using the lowest possible number for this purpose.
(3) Indicate by numbers the positions of other groups attached to the parent
chain. Alcohols containing two hydroxyl groups are called glycols. They
have both common names and IUPAC names. Physical properties
The compounds we have studied so far, the various hydrocarbons, are
nonpolar or nearly so, and have the physical properties that we might
expect of such compounds: the relatively low melting points and boiling
points that are characteristic of molecules with weak intermolecular forces;
solubility in non-polar solvents and insolubility in polar solvents like water.
Alcohols, in contrast, contain the very polar OH group. In particular, this
group contains hydrogen attached to the very electronegative element,
oxygen, and therefore permits hydrogen bonding (Sec. 1.19). The physical
properties show the effects of this hydrogen bonding. Let us look first at
boiling points. Among hydrocarbons the factors that determine boiling point
seem to be chiefly molecular weight and shape; this is to be expected of
molecules that are held together chiefly by van der Waals forces.
Alcohols, too, show increase in boiling point with increasing carbon
number, and decrease in boiling point with branching. But the unusual thing
about alcohols is that they boil so high: as Table 15.2 shows, much higher
than hydrocarbons of the same molecular weight, and higher, even, than
many other compounds of considerable polarity. How are we to account for
this? The answer is, of course, that alcohols, like water, are associated
liquids: their abnormally high boiling points are due to the greater energy
needed to break the hydrogen bonds that hold the molecules together.
Although ethers and aldehydes contain oxygen, they contain hydrogen that
is bonded only to carbon; these hydrogens are not positive enough to bond
appreciably with oxygen. Infrared spectroscopy (Sec. 13.4) has played a
key role in the study of hydrogen bonding. In dilute solution in a non-polar
solvent like carbon tetrachloriue (or in the gas phase), where association
between molecules is minimal, elhanol, for example, shows an O H
stretching band at 3640 cm" 1 . As the concentration
of ethanol is increased, this band is gradually replaced by a broader band
at 3350 cm"The bonding of hydrogen to the second oxygen weakens the O
H bond, and lowers the energy and hence the frequency of vibration. The
solubility behavior of alcohols also reflects their ability to form hydrogen
bonds. In sharp contrast to hydrocarbons, the lower alcohols are miscible
with water. Since alcohol molecules are held together by the same sort of
intermodular forces as water molecules, there can be mixing of the two
kinds of molecules: the energy required to break a hydrogen bond between
two water molecules or two alcohol molecules is provided by formation of a
hydrogen bond between a water molecule and an alcohol molecule.
This is true, however, only for the lower alcohols, where the OH group
constitutes a large portion of the molecule. A long aliphatic chain with a
smallOH group at one end is mostly alkane, and its physical properties
show this.
The change in solubility with carbon number is a gradual one: the first three
primary alcohols are miscible with water; -butyl alcohol is soluble to the
extent of 8 g per 100 g water; w-pentyl, 2 g; w-hexyl, 1 g; and the higher
alcohols still less.
For practical purposes we consider that the borderline between solubility
and insolubility in water occurs at about four to five carbon atoms for
normal primary alcohols.
Polyhydroxy alcohols provide more than one site per molecule for hydrogen
bonding, and their properties reflect this. The simplest glycol, ethylene
glycol, boils at 197. The lower glycols are miscible with water, and those
containing as many as seven carbon atoms show appreciable solubility in
water. (Ethylene glycol owes its use as an antifreeze e.g. Prestone to its
high boiling point, low freezing point, and high solubility in water.)