(Alcohols and Ethers)
Naming Alcohols:
Alcohols contain an OH group attached to a saturated carbon. The
common names for alcohols are based on the name of the alkyl group.
CH3OH
Methyl alcohol
CH3CH2OH
Ethyl alcohol
CH3CHOHCH3
Isopropyl alcohol
The systematic nomenclature for alcohols adds the ending -ol to the name
of the parent alkane and uses a number to identify the carbon that carries
the OH group. The systematic name for isopropyl alcohol, for
example, is 2-propanol.
CH3OH
Methanol
CH3CH2OH
Ethanol
CH3CHOHCH3
2-Propanol
Methanol, or methyl alcohol, is also known as wood alcohol because it
was originally made by heating wood until a liquid distilled. Methanol is
highly toxic, and many people have become blind or died from drinking
it. Ethanol, or ethyl alcohol, is the alcohol associated with "alcoholic"
beverages. It has been made for at least 6000 years by adding yeast to
solutions that are rich in either sugars or starches. The yeast cells obtain
energy from enzyme-catalyzed reactions that convert sugar or starch to
ethanol and CO2.
C6H12O6(aq)
2 CH3CH2OH(aq) + 2 CO2(g)
When the alcohol reaches a concentration of 10 to 12% by volume, the
yeast cells die. Brandy, rum, gin, and the various whiskeys that have a
higher concentration of alcohol are prepared by distilling the alcohol
produced by this fermentation reaction. Ethanol isn't as toxic as methanol,
but it is still dangerous. Most people are intoxicated at blood alcohol
levels of about 0.1 gram per 100 mL. An increase in the level of alcohol
in the blood to between 0.4 and 0.6 g/100 mL can lead to coma or death.
Testing Blood Alcohol Levels:
The method of choice for determining whether an individual is DUI
driving under the influence or DWI driving while intoxicated
is
the Breathalyzer, for which a patent was issued to R. F. Borkenstein in
1958. The chemistry behind the Breathalyzer is based on the reaction
between alcohol in the breath and the chromate or dichromate ion.
3 CH3CH2OH(g) + 2 Cr2O72-(aq) + 16 H+(aq)
3 CH3CO2H(aq) + 4 Cr3+(aq) + 11 H2O(l)
The instrument contains two ampules that hold small samples of
potassium dichromate dissolved in sulfuric acid. One of these ampules is
used a reference. The other is opened and the breath sample to be
analyzed is added to this ampule. If alcohol is present in the breath, it
reduces the yellow-orange Cr2O72- ion to the green Cr3+ ion. The extent to
which the color balance between the two ampules is disturbed is a direct
measure of the amount of alcohol in the breath sample. Measurements of
the alcohol on the breath are then converted into estimates of the
concentration of the alcohol in the blood by assuming that 2100 mL of air
exhaled from the lungs contains the same amount of alcohol as 1 mL of
blood.
Measurements taken with the Breathalyzer are reported in units of percent
blood-alcohol concentration (BAC). In most states, a BAC of 0.10% is
sufficient for a DUI or DWI conviction. (This corresponds to a bloodalcohol concentration of 0.10 grams of alcohol per 100 mL of blood.)
Ethanol is oxidized to CO2 and H2O by the alcohol dehydrogenase
enzymes in the body. This reaction gives off 30 kilojoules per gram,
which makes ethanol a better source of energy than carbohydrates (17
kJ/g), and almost as good a source of energy as fat (38 kJ/g). An ounce of
80-proof liquor can provide as much as 3% of the average daily caloric
intake, and drinking alcohol can contribute to obesity. Many alcoholics
are malnourished, however, because of the absence of vitamins in the
calories they obtain from alcoholic beverages.
Solubilities of Alcohols:
As a general rule, polar or ionic substances dissolve in polar solvents;
nonpolar substances dissolve in nonpolar solvents. As a result,
hydrocarbons don't dissolve in water. They are often said to be
immiscible (literally, "not mixable") in water. Alcohols, as might be
expected, have properties between the extremes of hydrocarbons and
water. When the hydrocarbon chain is short, the alcohol is soluble in
water. There is no limit on the amount of methanol (CH3OH) and ethanol
(CH3CH2OH), for example, that can dissolve in a given quantity of water.
As the hydrocarbon chain becomes longer, the alcohol becomes less
soluble in water, as shown in the table below. One end of the alcohol
molecules has so much nonpolar character it is said to
be hydrophobic (literally, "water-hating"). The other end contains an OH group that can form hydrogen bonds to neighboring water molecules
and is therefore said to be hydrophilic (literally, "water-loving"). As the
hydrocarbon chain becomes longer, the hydrophobic character of the
molecule increases, and the solubility of the alcohol in water gradually
decreases until it becomes essentially insoluble in water.
Solubilities of Alcohols in Water:
Solubility in
Water (g/100 g)
Formula
Name
CH3OH
methanol infinitely soluble
CH3CH2OH
ethanol
infinitely soluble
CH3(CH2)2OH propanol
infinitely soluble
CH3(CH2)3OH butanol
9
CH3(CH2)4OH pentanol
2.7
CH3(CH2)5OH hexanol
0.6
CH3(CH2)6OH heptanol
0.18
CH3(CH2)7OH octanol
0.054
CH3(CH2)9OH decanol
insoluble in water
Primary, Secondary, and Tertiary Alcohols:
Alcohols are classified as either primary (1), secondary (2), or tertiary (3)
on the basis of their structures.
Ethanol is a primary alcohol because there is only one alkyl group
attached to the carbon that carries the OH substituent. The structure of
a primary alcohol can be abbreviated as RCH2OH, where R stands for an
alkyl group. The isopropyl alcohol found in rubbing alcohol is a
secondary alcohol, which has two alkyl groups on the carbon atom with
the OH substituent (R2CHOH). An example of a tertiary alcohol
(R3COH) is tert-butyl (or t-butyl) alcohol or 2-methyl-2-propanol.
Phenols:
Another class of alcohols are the phenols, in which an OH group is
attached to an aromatic ring, as shown in the figure below. Phenols are
potent disinfectants. When antiseptic techniques were first introduced in
the 1860s by Joseph Lister, it was phenol (or carbolic acid, as it was then
known) that was used. Phenol derivatives, such as o-phenylphenol, are
still used in commercial disinfectants such as Lysol.
Boiling Points of Alcohols:
Water has an unusually high boiling point because of the hydrogen bonds
between the H2O molecules. Alcohols can form similar hydrogen bonds,
as shown in the figure below.
As a result, alcohols have boiling points that are much higher than
alkanes with similar molecular weights. The boiling point of ethanol, for
example, is 78.5ºC, whereas propane, with about the same molecular
weight, boils at -42.1ºC.
Alcohols as Acids and Bases:
Alcohols are Bronsted acids in aqueous solution.
CH3CH2OH(aq) + H2O(l)
H3O+(aq) + CH3CH2O-(aq)
Alcohols therefore react with sodium metal to produce sodium salts of the
corresponding conjugate base.
2 Na(s) + 2 CH3OH(l)
2 Na+(alc) + 2 CH3O-(alc) + H2(g)
The conjugate base of an alcohol is known as an alkoxide.
[Na+][CH3O-]
[Na+][CH3CH2O-]
sodium methoxide
sodium ethoxide
Preparation of Alcohols:
Alcohols can be prepared by adding water to an alkene in the presence of
a strong acid such as concentrated sulfuric acid. This reaction involves
adding an H2O molecule across a C=C double bond. Because these
reactions follow Markovnikov's rule, the product of the reaction is often a
highly substituted 2 or 3 alcohol.
Less substituted 1 alcohols can be prepared by substitution reactions that
occur when a primary alkyl halide is allowed to react with the OH- ion.
CH3CH2CH2Br + OH-
CH3CH2CH2OH + Br -
Ethers:
Alcohols (ROH) can be thought of as derivatives of water in which one of
the hydrogen atoms has been replaced by an alkyl group. If both of the
hydrogen atoms are replaced by alkyl groups, we get an ether (ROR).
These compounds are named by adding the word ether to the names of
the alkyl groups.
CH3CH2OCH2CH3 diethyl ether
Diethyl ether, often known by the generic name "ether," was once used
extensively as an anesthetic. Because mixtures of diethyl ether and air
explode in the presence of a spark, ether has been replaced by safer
anesthetics.
Alcohols versus Ethers:
There are important differences between both the physical and chemical
properties of alcohols and ethers. Consider diethyl ether and 1-butanol,
for example, which are constitutional isomers with the formula C4H10O.
CH3CH2OCH2CH3
CH3CH2CH2CH2OH
BP = 34.5ºC
BP = 117.2ºC
d = 0.7138 g/mL
d = 0.8098 g/mL
insoluble in water
soluble in water
The shapes of these molecules are remarkably similar, as shown in the
figure below.
The fundamental difference between these compounds is the presence
of OH groups in the alcohol that are missing in the ether. Because
hydrogen bonds can't form between the molecules in the ether, the boiling
point of this compound is more than 80ºC lower than the corresponding
alcohol. Because there are no hydrogen bonds to organize the structure of
the liquid, the ether is significantly less dense than the corresponding
alcohol.
Ethers can act as a hydrogen-bond acceptor, as shown in the figure below.
But, they can't act as hydrogen-bond donors. As a result, ethers are less
likely to be soluble in water than the alcohol with the same molecular
weight.
The absence of an OH group in an ether also has important
consequences for its chemical properties. Unlike alcohols, ethers are
essentially inert to chemical reactions. They don't react with most
oxidizing or reducing agents, and they are stable to most acids and bases,
except at high temperatures. They are therefore frequently used as
solvents for chemical reactions.
Compounds that are potential sources of an H+ ion, or proton, are often
described as being protic. Ethanol, for example, is a protic solvent.
H3O+(aq) + CH3CH2O-(aq)
CH3CH2OH(aq) + H2O(l)
Substances that can't act as a source of a proton are said to be aprotic.
Because they don't contain an OH group, ethers are aprotic solvents.
Ethers can be synthesized by splitting out a molecule of water between
two alcohols in the presence of heat and concentrated sulfuric acid.
H+
2 CH3CH2OH
CH3CH2OCH2CH3
They can also be formed by reacting a primary alkyl halide with an
alkoxide ion.
CH3CH2CH2Br + CH3O-
CH3CH2CH2OCH3 + Br-
by : Afrah Eid AL.azmi
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