Chemistry 1027A/B Lecture Notes - Substitution Reaction, Amphoterism, Lone Pair
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Key Concepts In Organic Chemistry: Alcohols, Ethers & Thiols
-Alcohols have the general formula R-OH. The O atom is always ignored in calculations of units of
unsaturation. The O in alcohols is sp3 hybridized (two sp3 orbitals are occupied by non-bonding pairs).
Thiols have the general formula R-SH. The S atom is always ignored in calculations of units of
unsaturation. In alcohol/thiol naming, the OH/SH group always takes priority over π bonds. Alcohols are
classed as 1°, 2° or 3° depending on the type of sp3 C atom to which the OH group is bonded.
-Hydrogen bond refers to the attractive force between an Hδ+ of one functional group and the δ- of the
lone pair on the O (or other) atom of another. A hydrogen bond is not a true (σ) bond, but is instead a
coulombic attraction between partial charges on atoms in neighbouring molecules.
-Alcohols tend to have very high boiling points for their size. OH groups greatly increase the solubility of
organic molecules in water. One of the most noteworthy physical feature of thiols is their incredibly
-Like H2O, alochols and thiols are amphiprotic, they can function as acids (in the presence of sufficiently
strong bases) or as bases (in the presence of sufficiently strong acids). Alcohols, like water, also react
with Group 1 and 2 metals to form alkoxide salts, with the liberation of hydrogen:
ROH + Na RO-Na+ + 1/2H2
-The conversion of alcohols to alkyl halides is a substitution reaction that proceeds by an SN1 or an SN2
mechanism, depending on the structure of the alcohol. In the reaction of alcohol with HX (X = Cl, Br, I),
tertiary alcohols react by an SN1 mechanism, but only after the hydroxyl group is protonated to make an
oxonium ion. This converts the OH to an excellent LG, OH2. Primary and secondary alcohols also give
alkyl halides on reaction with HX, but an SN1 mechanism is energetically unfavourable and an SN2
mechanism occurs instead.
-In the reaction of alcohol with thionyl chloride, SOCl2, better yields are obtained from 1° or 2° (but not
3°) because coproducts bubble out of solution, making the reaction irreversible. It occurs by SN2
-In the dehydration of alcohols to alkenes, heating (Δ) is required (as in Lab #3). This reaction undergoes
elimination to form double bonds (especially favoured) only when stable double bonds are formed (the
product with the most alkyl groups will form).
-1° and 2° alcohols are easily oxidized to carbonyl compounds. Because oxidation removes an H atom
from both the O atom and the C atom, only 1° and 2° alcohols react; 3° alcohols cannot be oxidized to
simple carbonyl compounds. PCC is an oxidizing agent that has been developed to oxidize 1° alcohols to
aldehydes only, thus solving the practical problem of 1° alcohol oxidation.
-Thiols are not oxidized to thiones (C = S), but instead to disulfides (RS-SR). Disulfides can also be
reduced back to thiols through the use of reducing agents.
-The general formula for ethers is R-O-R. As in alcohols, the O atom is ignored for sites of unsaturation
calculations. Ethers contain 2 polar bonds, and have a small dipole moment, but cannot H-bond by
-In the preparation of ethers reaction, an SN2 attack of alkoxide nucleophiles on 1° and 2° alkyl halides is
required: RO- + R’ – X ROR’ + X-
-Epoxides are 3-membered ring ethers. Epoxides are prepared from alkenes by reaction with a peroxide.
The addition of the O atom to the C = C is stereospecific and gives only syn addition.
-Ethers are generally quite unreactive, but epoxides are very reactive and engage in the ring-opening
reaction, which proceeds with the anti-addition of the OH groups. Good nucleophiles like NH3 and HS-
can also add to epoxides by an SN2 process in an anti-manner.
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