Ch. 5: Alcohols Flashcards
what is the general formula for alcohols?
ROH
what does a hydroxyl group refer to?
the functional group -OH
how are alcohols named in the IUPAC system?
by replacing -e ending of the root alkane with the ending -ol
what is an alternative common naming practice?
to name the alkyl group as a derivative, followed by alcohol (like ethyl alcohol)
how does naming work when the alcohol is the highest-priority functional group? when it’s not the highest priority functional group?
YES: the carbon atom attached to it receives the lowest possible number
NO: it is named as a substituent, with the prefix hydroxy-
defn: phenol
compounds that have hydroxyl groups attached to aromatic rings
why are the hydroxyl hydrogens of phenols particularly acidic?
due to the aromatic nature of the phenol ring, which allows for the resonances stabilization of the negative charge on oxygen, stabilizing the anion
what do ortho (o)
meta (m)
and para (p) indicate as prefixes? when are they used?
when benzene rings contain two substituents, their relative positions must be indicated
ortho (o): two groups on adjacent carbons
meta (m): two groups separated by a carbon
para (p): two groups on opposite sides of the ring
what else does hydrogen bonding increase other than melting and boiling points?
solubility in water
why are the melting and boiling points of alcohols significantly higher than their analogous hydrocarbons?
because they are capable of intermolecular hydrogen bonding
does the number of hydroxyl groups in a compound affect the hydrogen bonding or the boiling point?
yes, both!
more hydroxyl groups –> greater degree of hydrogen bonding –> higher boiling point
explain how hydrogen bonding works in the context of a hydroxyl group (3)
- the electronegative oxygen atom pulls electron density away from the less electronegative hydrogen atom
- this generates a slightly positive charge on the hydrogen and slightly negative charge on the oxygen
- then, the partially positive hydrogen of one molecule electrostatically attracts the partially negative oxygen of another molecule, generating a noncovalent bonding force (a hydrogen bond)
why and into what do alcohols dissociate?
they dissociate into protons and alkoxide ions
because the hydroxyl hydrogen is weakly acidic
why can phenols form salts with inorganic bases such as NaOH, but nonaromatic alcohols cannot?
because phenols are much more acidic than nonaromatic alcohols
does the presence of other substituents on the ring affect acidity, boiling point, and melting point of phenols? if yes, how so?
yes, significantly so
electron-withdrawing groups INCREASE acidity
electron-donating groups DECREASE acidity
how does the presence of more alkyl groups on nonaromatic alcohols affect the acidity? why?
more alkyl groups –> produces LESS acidic molecules
because alkyl groups donate electron density, they destabilize a negative charge
alkyl groups also help stabilize positive charges
what are the 3 main reactions we will see on the MCAT for alcohols?
- oxidation
- preparation of mesylates and tosylates
- protection of carbonyls by alcohols
what does PCC oxidize a primary alcohol to and why?
what does PCC stand for?
PCC = pyridinium chlorochromate
oxidized to aldehydes
stops there because PCC lacks the water necessary to hydrate the easily hydrated aldehyde
what are aldehydes oxidized to?
what comes after that?
geminal diols –> carboxylic acids
what are secondary alcohols oxidized to? by what?
to ketones
by PCC or any stronger oxidizing agent
why can’t tertiary alcohols be oxidized?
they are already as oxidized as they can be without breaking a carbon-carbon bond
what is the result of oxidation of primary alcohols with a strong oxidizing agent? what about secondary alcohols?
primary: a carboxylic acid!
secondary: a ketone
what are 2 common examples of chromium-containing oxidizing agents?
what happens to chromium in the process of acting as an oxidizing agent?
sodium dichromate (Na2Cr2O7)
potassium dichromate (K2Cr2O7)
Chromium(VI) is reduced to chromium(III)
what is an even stronger chromium-containing oxidizing agent?
chromium trioxide, CrO3
process + result: Jones oxidation
process: when chromium trioxide is dissolved with dilute sulfuric acid in acetone
result: this reaction oxidizes primary alcohols to carboxylic acids and secondary alcohols to ketones
hydroxyl groups of alcohols are fairly poor leaving groups for nucleophilic substitution reactions. what can they be turned into to make much better leaving groups? how?
they can be protonated or reacted to form mesylates and tosylates
defn: mesylate
a compound containing the functional group -SO3CH3, derived from methanesulfonic acid
how are mesylates prepared?
using methylsulfonyl chloride and an alcohol in the presence of a base
defn: tosylate
a compound containing the functional group SO3C6H4CH3, derived from toluene-sulfonic acid
how are tosylates produced?
by the reaction of alcohols with p-toluenesulfonyl chloride, forming esters of toluenesulfonic acid
what other function to mesyl and tosyl groups have?
they can solve as protecting groups when we do not want alcohols to react
why and how are mesyl and tosyl groups protective?
they will not react with many of the other reagents that would attack alcohols, especially oxidizing agents
so, reacting an alcohol to form a mesylate to tosylate is sometimes performed before multistep reactions in which the desired products do not derive from the alcohol
how can alcohols be used as protecting groups for other functional groups themselves?
aldehydes and ketones can be reacted with 2 equivalents of an alcohol or a diol, forming acetals or ketals
defn: acetal
defn: ketal
acetal: primary carbons with two -OR groups and a hydrogen atom
ketal: secondary carbons with two -OR groups
why are acetals and ketals good protecting groups?
carbonyls are very reactive with strong reducing agents like LiAlH4, but acetals and ketals do not
process: deprotection
the acetal or ketal can be converted back to a carbonyl with aqueous acid
defn + char (3): quinone
produced from: treatment of phenols with oxidizing agents (2,5-cyclohexadiene-1,4-diones)
- resonance-stabilized electrophiles due to the conjugated ring system
- not necessarily aromatic because they lack classic aromatic conjugated ring structure (some do, but not always)
- biochemically serve as electron acceptors (spec. in ETC)
how are quinones named?
by indicating the position of the carbonyls numerically and adding quinone to the name of the parent phenol
common name + use: phylloquinone
vitamin K1
important for photosynthesis and the carboxylation of some of the clotting factors in blood
common name: menaquinone
vitamin K2
source + char (3): hydroxyquinone
oxidized from quinones
- share the same ring and carbonyl backbone as quinones, but differ by the addition of one or more hydroxyl groups
- many have biological activity, some used in medication synthesis
- bc of resonance: behave like quinones with electron-donating groups (slightly less electrophilic, but still very reactive)
defn: hydroquinone vs. hydroxyquinone
hydroquinone: a benzene ring with 2 hydroxyl groups
hydroxyquinone: contains 2 carbonyls and a variable number of hydroxyl groups
how do you name hydroxyquinones?
the position of the hydroxyl groups is indicated by a number
the total number of hydroxyl groups is indicated by a prefix if there is more than one, with the substituent name hydroxy
defn + aka + char (3): ubiquinone
- a biologically active quinone
- coenzyme Q
- vital electron carrier associated with complexes I, II, and III of the ETC
- the most oxidized form that this molecule takes physiologically
- lipid soluble bc of long alkyl chain (so it can act as an electron carrier within the phospholipid bilayer)
how does ubiquinone turn into ubiquinol?
how does this play a role biologically?
we can reduce ubiquinone to ubiquinol upon the acceptance of electrons
the oxidation-reduction capacity allows the molecule to perform physiologically in terms of electron transport
what are 3 other molecules that undergo redox reactions as a part of their normal function (and accept and donate electrons readily, like ubiquinone)?
NADH
FADH2
NADPH
