The Alcohols section provides High Yield Information needed to succeed in the MCAT exam and medical school for college students and medical students interested in the medical field.
Description and Properties
- Alcohols have the general formula ROH, with the function group – OH being called the hydroxyl
Nomenclature
- Replace the –e ending of the root alkane with the ending –ol.
- If the alcohol is the highest-priority functional group, then the carbon atom attached to it receives the lowest possible number.
- Can also be named using common names: Name the alkyl group as a derivative followed by alcohol.
- When the alcohol is not the highest priority functional group, it is named as a substituent with the prefix: hydroxyl-
- Phenols: hydroxyl groups attached to aromatic rings
- Hydroxyl hydrogens are acidic due to resonance within the phenol ring
- When benzene rings contain two substituents, their relative position must be indicated
- Two groups on adjacent carbons is called ortho- or simple o-
- Two groups separated by a carbon are called meta- or m-
- Two groups on opposite sides of the ring are called para- or p-
Physical Properties
- Capable of intermolecular hydrogen bonding which results in significantly higher melting and boiling points than those of analogous hydrocarbons.
- If more than one hydroxyl group present, greater degree of hydrogen bonding is present.
- Hydrogen bonding occurs when hydrogen atoms are attached to highly E.N atoms like Nitrogen, oxygen, or fluorine. And is a result of the extreme polarity of these bonds.
- The partially positive hydrogen of one molecule electrostatically attracts the partially negative oxygen of another molecule which generates a noncovalent bond known as a hydrogen bond.
- Hydroxyl hydrogen is weakly acidic. Alcohols can dissociate into protons and alkoxide ions.
- Phenols are more acidic than those of other alcohols. This is due to aromatic nature of ring which allows for resonance to stabilize the negative charge on the oxygen.
- Slightly soluble in water due to hydrogen bond
- Can also form salts with inorganic bases such as NaOH since they are more acidic.
- Presence of other substituents has drastic effects on the acidity, boiling point, and melting points of phenols
- Electrons-withdrawing substituents increase acidity, while electron-donating groups decrease acidity.
- More alkyl groups in nonaromatic alcohols produces less acidic molecules, since these alkyl groups donate electron density.
Reaction of Alcohols
Oxidation Reactions
- Primary alcohols can be oxidized to aldehydes but only by pyridinium chlorochromate (PCC).
- Reactant stops after the primary alcohol is converted to an aldehyde because PCC lacks the water necessary to hydrate the aldehyde.
- Other oxidizing agents can be used to rapidly hydrate aldehydes to form germinal diols (1,1-diols) which are then oxidized to a carboxylic acid.
- Secondary alcohols (2 carbons attached to a carbon) can be oxidized to ketones by PCC or any stronger oxidizing agent.
- Tertiary alcohols cannot be oxidized because they are already as oxidized as they can be without breaking a carbon-carbon bond
- Oxidation of a primary alcohol with a strong oxidizing agent like chromium (VI) will produce a carboxylic acid.
- Chromium (IV) is reduced to chromium (III)
- Will also oxidize secondary alcohols to ketones.
- Strongest oxidizing agent is called the Jones oxidation (CrO3 dissolved with dilute sulfuric acid in acetone)
Mesylates and Tosylates
- Hydroxyl groups of alcohols are poor leaving groups for nucleophilic substitution reactions.
- Protonated hydroxyl groups are called mesylates and tosylates and are much better leaving groups.
- Mesylate: compound containing the functional group –SO3CH3 which is derived from methanesulfonic acid.
- Tosylates: contain the functional group –SO3C6H4CH3 which is derived from toluenesulfonic acid
- These molecules can also act as protecting groups for when we don’t want an alcohol to react.
- Groups do not react with many of the other reagents that would attack alcohols, especially oxidizing agents. Thus reacting an alcohol to form a T or M is performed before reaction in which the desired products do not derive from alcohols.
Protecting Groups
- Alcohols themselves can also be used as a protecting groups for other functional groups
- Aldehydes and ketones can be reacted with two equivalents of an alcohol or a diol, which forms ketals (secondary carbons with two –OR groups) or acetals (primary carbons with two –OR groups).
- Help protect the aldehydes or ketone from reaction with strong reducing agents.
- Deprotection: Acetal or ketal can be reverted back to a carbonyl with aqueous acid
Reactions of Phenols
Quinones and Hydroxyquinones
- Treatment of phenols with oxidizing agents produces compounds called quinones.
- Named by indicated the position of the carbonyls numerically and adding quinone to the name of the parent phenol
- Resonance-stabilized electrophiles, but are not always aromatic since they can sometime lack the classic aromatic conjugated structure.
- Serve as electron acceptors in biochemical function (think Vitamin K1 in electron transport chain of aerobic respiration).
- Vitamin K2 belongs to a class of molecules called menquinones.
- Quinones can be further oxidized to form hydroxyquinones
- These share the same ring and carbonyl backbone as quinones, but have one or more additional hydroxyl group.
- Behave like quinones with electron-donating groups that make them slightly less electrophilic.
- Naming: position of hydroxyl groups is indicated by a number and the total number of hydroxyl groups is indicated by a prefix with the substituent name hydroxyl-.
Ubiquinone
- Biologically active quinone and is sometimes called coenzyme Q and is a vital electron carrier associated with complexes I, II, and III of the electron transport chain.
- Can be reduced to ubiquinol upon the acceptance of electrons, which is the basis of its physiological function.
- Long alkyl chain allows it to be lipid soluble which allows it to act as an electron carrier within the phospholipid bilayer.
