Alcohols, Ethers and Thiols
Alcohols (R-OH)
• Alcohols are organic compounds that contain
the –OH (hydroxyl) functional group attached
to the hydrocarbon skeleton (alkyl or aryl)
• Simplest alcohol is methanol (CH3OH)
Chemistry, Raymond Chang, 10th edition, McGraw-Hill
Alcohols – Hydroxyl Functional Group
• Many important
biological molecules
have one or more
hydroxyl groups (sugars,
fats, proteins)
• Hydroxyl group is
involved in many
reactions in biological
systems such as
oxidation, reduction,
dehydration
Biological production of ethanol – fermentation of sugar
glucose
serine
Naming Alcohols
• IUPAC nomenclature rules for alcohols:
1) Find the parent compound name, which is the
longest continuous carbon chain containing the
hydroxyl group
2) Replace “-e” with “-ol”
- For example;
ethane
ethan(e) + ol
3) Number the parent chain so that the carbon
attached to the hydroxyl has the lowest possible
number
4) Add substituent names as prefixes to the main
name
5) Alcohols with two hydroxyls are named “-diols”
•
Common names are used for some alcohols, with
the name of the alkyl group: isopropyl alcohol,
ethylene glycol, ethyl alcohol)
http://www.ck12.org/book/CK-12-Chemistry-Second-Edition/r13/section/25.4/
Physical Properties of Alcohols
• Polar molecules (presence of highly electronegative oxygen atom)
• Form hydrogen bonds between themselves and with water
molecules
• They have high melting and boiling points especially due to
hydrogen binding
CH3CH2CH2CH3 (butane) MW = 58, b.p. = -0.5 °C
CH3CH2CH2OH (1-propanol) MW = 60, b.p. = 97.2 °C
• Small alcohols (1-4 carbons) are very soluble in water
• As the hydrocarbon part (nonpolar) becomes larger relative to
hydroxyl (polar) part, alcohols become less soluble (alcohols with 7
or more carbons are almost insoluble)
http://www.chemhume.co.uk/ASCHEM/Unit%202/Ch11%20Alcohols/images/hydrogen_bonding_ethanol_water.jpg
Medically Important Alcohols
• Methanol (CH3OH): Used mainly as a solvent, also
called wood alcohol, toxic, can cause blindness and
even death if ingested
• Ethanol (C2H5OH): Used as solvent, beverage and raw
material for synthesis of more complex organic
compounds, produced by fermentation of sugar
• 2-Propanol (isopropyl alcohol): used as disinfectant
(before and after injection/blood test), as an astringent
(skin-drying agent), as industrial solvent / toxic when
ingested
• 1,2,3-Propanetriol (glycerol): viscous liquid, very
soluble in water, used in cosmetics and pharmaceutical
industry
Classification of Alcohols
• Alcohols are classified as primary (1°),
secondary (2°) and tertiary (3°)
http://www.chem.shef.ac.uk/chm131-2003/cha03kej/alcohols.html
Reactions of Alcohols
1) Preparation of alcohols:
2) Dehydration of alcohols:
3) Oxidation reactions:
Reactions of Alcohols
1) Preparation of alcohols:
a) Hydration reaction: addition of water to alkenes
(carbon-carbon double bond) – follows
Markovnikov rule:
b) Hydrogenation (reduction) of aldehydes and
ketones (by metal catalyst)
http://www.chem.latech.edu/~deddy/chem121/Alcohols.htm
Reactions of Alcohols
2) Dehydration of alcohols (Elimination of Water):
• Alcohols undergo loss of water (dehydration)
when they are heated with acid
R-CH2-CH2-OH + H2SO4 ---> R-CH=CH2
• Reverse process of hydration reaction
• Predicting products of alcohol dehydration Zaitsev’s rule- more substituted alkene is
preferred
http://www.masterorganicchemistry.com/2012/08/31/elimination-reactions-2-zaitsevs-rule/
Reactions of Alcohols
3) Oxidation Reactions:
• An oxidation reaction
involves gain of oxygen or
loss of hydrogen
• Alcohols can be oxidized
to aldehydes, ketones
and carboxylic acids
• Most common oxidizing
agents are potassium
permanganate (KMnO4)
and chromic acid (H2CrO4)
• Tertiary alcohols can’t be
oxidized
http://www.wikipremed.com/03_organicmechanisms.php?mch_code=030207_050
Ethers (R-O-R)
• Ethers are organic compounds that contain
the –OR (alkoxy) functional group attached to
the hydrocarbon skeleton
• -OH group in alcohols is replaced by –OR (alkyl
or aryl group instead of hydrogen)
• In other words, ethers are compounds in
which two alkyl (or aryl) groups are linked by a
bridging oxygen
Naming Ethers
• In IUPAC system, -OR substituent is named as
alkoxy group (similar to hydroxy)
CH3-O- (methoxy), CH3CH2-O- (ethoxy)
• Ethers are named by placing the two alkyl
group names in front of the word “ether”
• The two names are placed either
alphabetically or by size (smaller first)
examples: dimethyl ether, ethyl methyl ether,
ethyl propyl ether
Physical Properties of Ethers
1) Ethers are relatively polar molecules (but not as
polar as alcohols)
2) Unlike alcohols, they can’t make hydrogen bonds
with itself – so lower mp and bp compared to
alcohols
3) They can make hydrogen bonds with water –
more soluble than alkanes but less than alcohols
http://www.masterorganicchemistry.com/2010/10/25/3-trends-that-affect-boiling-points/
Chemical Properties and Preparation
• Ethers are moderately inert (not very reactive)
• Extremely volatile and flammable (easily
oxidized in the air), requires great care
• Prepared by a dehydration reaction from two
alcohol molecules in the presence of acid and
heat:
R-OH + R-OH  R-O-R + H2O (heat/acid)
(heated with sulfuric acid at 140 °C)
Medical Importance of Ethers
• Diethyl ether was the first
general anesthetic used (in
1800s) – used in surgeries
• How does it work?
- Diethyl ether accumulate in the
lipid material of the nerve cells
in the central nervous system
and interfere with nerve
impulse transmission
- This leads to analgesia,
lessened perception of pain
and unconsciousness
“William Morton, a Boston dentist,
introduced diethyl ether into surgical
practice in 1846. This painting shows
an operation in Boston in 1846 in
which diethyl ether was used as an
anesthetic.
Inhalation of ether vapor produces
unconsciousness by depressing the
activity of the central nervous system”
http://2012books.lardbucket.org/books/introduction-to-chemistry-general-organic-and-biological/s17-08-ethers.html
Thiols
• Thiols contain sulfhydryl group (-SH), similar to
alcohols, but sulfur instead of oxygen
• IUPAC rules for naming thiols: similar to
alcohols, except “-thiol” is added at the end of
the name of the parent alkane
• Thiol compounds have strong smells (onion,
garlic etc.)
http://2012books.lardbucket.org/books/beginning-chemistry/s20-05-other-functional-groups.html
Importance of Thiols in Biology
• The amino acid “cysteine” is a thiol
cysteine
• Cysteine plays an important role in the structure of
proteins (disulfide bonds)
• Coenyzme A (acetyl group carrier in many
biochemical reactions) is also a thiol
Coenzyme A