Organic Compound
＊Definition: Alcohol is organic compound in which a hydroxyl group (-OH) is
bond to a carbon atom. The general formula for a simple alcohol containing no rings
is CnH(2n+1)OH.
＊Classification: Three major subsets of alcohols- 'primary' (1°), 'secondary' (2°) and
'tertiary' (3°), based upon the number of carbons the C-OH carbon is bonded to. A
primary (1°) alcohol is one in which the carbon atom with the OH group is attached
to one other carbon atom. Its general formula is RCH2OH. A secondary (2°)
alcohol is one in which the carbon atom (in red) with the OH group is attached to two
other carbon atoms (in blue). Its general formula is R2CHOH. A tertiary (3°)
alcohol is one in which the carbon atom with the OH group is attached to three other
carbon atoms. Its general formula is R3COH. Is isobutyl alcohol primary, secondary,
or tertiary? Primary
Methanol and ethanol: Methanol was formerly obtained by the distillation of wood,
and was called "wood alcohol". It is now a cheap commodity chemical. Aside from
ethanol’s major use in alcoholic beverages, ethanol is also used (though highly
controlled) as an industrial solvent and raw material.
＊ Nomenclature: Common Names: Alcohols with one to four carbon atoms are
frequently called by common names, in which the name of the alkyl group is
followed by the word alcohol:
IUPAC System: alcohols are named by changing the ending of the parent alkane name
to -ol. 1. The longest continuous chain (LCC) of carbon atoms containing the OH
group is taken as the parent compound. The chain is numbered from the end nearest
the OH group. 2. The number that indicates the position of the OH group is placed
before the name of the parent hydrocarbon, and separated by a dash. The -e ending of
the parent alkane is replaced by the suffix -ol. Substituents are named and numbered
as in alkanes. In cyclic alcohols, the carbon atom bearing the OH group is designated
C1, but the 1 is not used in the name.3. If more than one OH group appears in the
same molecule (polyhydroxy alcohols), suffixes such as -diol and -triol are used. In
these cases, the -e ending of the parent alkane is retained.
Draw the structure for the compound. 2-hexanol: 1. The ending -ol indicates an
alcohol (the OH functional group), and the hex- stem tells us that there are six carbon
atoms in the LCC. We start by drawing a chain of six carbon atoms:
–C–C–C–C–C–C–. 2. The 2 indicates that the OH group is attached to the second
carbon atom. 3. Finally, we add enough hydrogen atoms to give each carbon atom
four bonds.
1.
1,5-pentanediol
2. 3-pentanol
＊Properties: Alcohol is a general term denoting a family of organic chemicals with
common properties. Members of this family include ethanol, methanol, isopropanol
and others.
Most of the common alcohols are colourless liquids at room temperature.
The hydroxyl group generally makes the alcohol molecule polar and soluble in
water, while the non-polar hydrocarbon chain tends to make alcohols insoluble.
Hydrogen bonding raises the boiling point of alcohols. This is due to the
combined strength of so many hydrogen bonds forming between oxygen atoms of one
alcohol molecule and the hydroxy H atoms of another. The longer the carbon chain in
an alcohol is, the lower the solubility in polar solvents and the higher the solubility
in nonpolar solvents.
Practice：Arrange according to increasing boiling point. (start with lowest boiling
point)1. CH4, CH3OH, CH3CH3
Arrange according to increasing solubility (start with lowest solubility)
2. CH4, CH3CH2Cl, CH3CH2OH
Solution: CH4, CH3CH3, CH3OH ;CH3CH2Cl, CH4, CH3CH2OH
Solubility: The small alcohols are completely soluble in water. However, solubility
falls as the length of the hydrocarbon chain in the alcohol increases. The -OH end of
the alcohol molecules can form new hydrogen bonds with water molecules, but the
hydrocarbon “tail” doesn‘t form hydrogen bonds. So quite a lot of the original
hydrogen bonds being broken are replaced by London force.
Chemical Properties: Deprotonation: Alcohols can behave as weak acids,
undergoing deprotonation. e.g. 2 CH3CH2-OH + 2 Na → 2 CH3-CH2-O−Na+
Toxicity: All alcohols are poisonous. Ethanol is no exception. While infrequent
consumption of ethanol in small quantities may be harmless or even beneficial, larger
doses result in drunkenness, and even can cause severe health problems, such as
liver and brain damage.
Exercises:
1. Name each alcohol and classify it as primary, secondary, or tertiary.
a. CH3CH2CH2CH2CH2CH2OH
1-hexanol; primary
b.
3-hexanol; secondary
c.
3,3-dibromo-2-methyl-2-butanol; tertiary
2. Draw the structure for each alcohol.
a. 3-hexanol
b. 3,3-dimethyl-2-butanol
c. cyclobutanol
3. What is the general formula of a simple alcohol?
CnH(2n+1)OH
4. How to determine which kind of subset is an alcohol?
Based upon the number
of carbons the C-OH carbon is bonded to.
5. What makes the alcohol molecule polar ? The hydroxyl group (OH)
6. Which alcohol is less poisonous than other alcohols? Ethanol