10A. Alcohols

Question 1

Alcohols

Answer 1

• compounds containing an -OH group (hydroxyl) bonded to a tetrahedral carbon atom

C - O - H

• classified as either primary (1^o), secondary (2^o), or tertiary (3^o) depending on the number of carbon atoms bonded to the carbon bearing the -OH group
• structurally, alcohols may be viewed in two ways: as hydroxy derivatives of alkanes, where one hydrogen of the alkane has been replaced by a hydroxyl group; or as alkyl derivatives of water, where one hydrogen of water has been replaced by an alkyl group

Question 2

Naming

Answer 2

• similar to alkenes + alkynes, except that the parent chain contains the -OH group + is numbered from the end closest to the -OH group
• for cyclic alcohols, the carbon bearing the -OH group is always number 1
• the location of the -OH group is shown + the -e on the end of the parent alkane is replaced by -ol
• to dreive common names for alcohols, the alkyl group bonded to the -OH group is named, followd by ‘alcohol’
• a compound containing two hydroxyl groups is known as a ‘diol’, ‘triol’ for three and so on.When naming such compounds, diol, triol etc is simply suffixed to the name of the parent alkane
• glycol is the common name for compound with hydroxyl groups on adjacent carbons

Question 3

Physical Properties

Answer 3

• most important is the polarity of the -OH group
• because of the large difference in electronegativity between oxygen + carbon + between oxygen + hydrogen, both the C-O + O-H bonds are polar covalent, resulting in partial positive charges on the participatingcarbon + hydrogen atoms + a partial negative charge on the oxygen atom
• this polarity results in hydrogen bonding between alcohol molecules an the liquid state + therefore alcohols have higher boiling points than alkanes, alkenes + alkynes of similar molecular weight
• also, as molecular weight increases so to do boiling points due to increased London dispersion forces
• lower molecular weight alcohols are much more soluble in water than are hydrocarbons of similar molecular weight because of the hydrogen bonding between water and alcohol molecules. However, as molecular weight increase + the size of the hydroxyl group relative to the hydrocarbon portion of the molecule decrease, so too does the solubility in water
• each hydroxyl group can participate in up to three hydrogen bonds - one with the hydrogen + two with oxygen

Question 4

Reactions

Answer 4

• two key reactions:dehydration to alkenes, and oxidation to aldehydes, ketones + carboxylix acids
• alcohols have about the same pK_a value as water + therefore approximately the same pH as pure water

Question 5

Dehydration to alkenes

Answer 5

• just as acid-catalyzed hydration of alkenes produce alcohols, acid-catalyzed dehydration of alcohols produce alkenes
• hydration-dehydration reactions are reversible, therefore alkene hydration + alcohol dehydration are competing reactions reaching equilibrium
• in the process of dehydration, the -OH group is removed from one carbon + a hydrogen is removed from an adjacent carbon as the alcohol is heated in a concentrated acid, commonly phosphoric or sulfuric acid
• the conditions required to bring about dehydration - temperature + acid concentration - are closely related to the structure of individual alcohols
• primary alcohols are the most difficult to dehydrate, requiring highly concentrated acid + temperatures up to 180oC
• secondary alcohols require less acid concentration + lower temperatures
• tertiary alcohols are the easiest to dehydrate with mildly concentrated acids + temperatures not much above room temperature
• certain alcohols dehydrate to yield isomeric alkenes. In this case, the isomer with the greater number of alkyl groups on the double bond will predominate as it has the greater stability
• using le Chatelier’s principle, by altering the reaction conditions, the hydration-dehydration equilibrium can be used to prepare both alcohols + alkenes, each in high yields
• large amounts of water ie. using dilute aqueous acid, will favour alcohol formation, whereas water scarcity ie. using concentrated acid, or conditions where water is removed ie. heating the reaction mixture above 100oC, will favour alkene production

Question 6

Oxidation to aldehydes, ketones + carboxylic acids

Answer 6

• depending on the conditions, all alcohols can be oxidized
• primary alcohols*
• primary alcohols can be oxidized first to an aldehyde by losing hydrogen atoms, + then to a carboxylic acid by gaining an oxygen atom
• the reagent most commonly used for oxidation of a primary alcohol is potassium dichromate, K₂Cr₂O₇, dissolved in aqueous sulfuric acid
• although the usual product of oxydation of a primary alcohol is carboxylic acid, it is possible to stop the oxidation at the aldehyde stage by distilling the mixture and removing the aldehyde, which usually has a lower boiling point than either the primary alcohol or the carboxylic acid
• secondary alcohols*
• secondary alcohols can be oxidsed to ketones by using potassium dichromate as the oxidising agent
• the reaction usually stops at the ketone stage because further oxidation requires the breaking of a carbon-carbon bond
• tertiary alcohols*
• tertiary alcohols can be oxidised but only under very forceful conditions
• the carbon bonded to the -OH group is also bonded to three carbon atoms + therefore cannot form a carbon- oxygen double bond
• also there is no hydrogen attached to the carbon bonded to the -OH group + therefore no second hydrogen to be given up

Question 7

Phenol

Answer 7

• compound that contains an -OH group bonded to a benzene ring
• substituted phenols are named either as derivatives of phenol or by common names
• phenols are widely distributed in nature