hydroxy compounds Flashcards
why do ethers, constitutional isomers of alcohols, have none of the reactions of alcohols?
ethers do not contain the -OH group -> have none of the reactions of alcohols, and also do not form intermolecular hydrogen bonds -> hence they inert compounds with lower boiling points than their isomeric compounds
why do alcohols & phenols have significantly higher boiling points than those of their corresponding isoelectronic hydrocarbons?
- strength of dispersion forces in the alcohols & in the alkane is about the same due to the similar sizes of their electron clouds (since the compounds being compared are isoelectronic - same no. of electrons)
- but more energy is required to overcome the additional intermolecular hydrogen bonding in alcohols, which is stronger than the dispersion forces in alkanes
same for phenol - presence of H bonds makes mp/bp higher than hydrocarbons w similar no. of electrons
why does boiling point of alcohols incraese with increasing carbon number?
- increasing carbon number -> increasing alkyl chain length
- strength of intermolecular hydrogen bonding remains largely similar for the alcohols with differing alkyl chain lengths (bc only -OH group can form H bonds, and each alcohol being compared only has 1 -OH group)
- but the size of the electron cloud increases as alkyl chain length increases -> more energy is required to overcome stronger dispersion forces between alcohol molecules
why are alcohols more soluble in water compared to their corresponding alkanes?
alcohols are more soluble as they can form hydrogen bonds with water molecules, while alkanes cannot
- energy released when forming H bonding between alcohols & water molecules is comparable to the energy required to overcome H bonding between water molecules and H bonding between alcohol molecules
- energy released when forming dispersion forces between alkanes & water molecules is insufficient to overcome H bonding between water molecules & dispersion forces between alkane molecules
phenols are only moderately soluble in water bc of their non-polar aryl group which limits their ability to form H bonds w water, but they dissolve completely when warmed
why does the solubility of alcohols decrease with increasing carbon number?
- increasing carbon number -> increasing length of non-polar alkyl chain -> strength of dispersion forces between alcohol molecules increases
- energy released from hydrogen bonding formed between the alcohol molecule & water is increasingly less able to overcome the increasingly stronger dispersion forces between alcohol molecules and existing hydrogen bonding in water
- hydrogen bonding in water is also disrupted by the increasing non-polar alkyl chain
how can alcohols be prepared?
- electrophilic addition of water or steam to alkenes (conc. H2SO4, cold, H2O)
- nucleophilic substitution of halogenoalkanes (dilute NaOH, heat)
- reduction of aldehydes & ketones (LiAlH4 in dry ether, room temperature)
- reduction of carboxylic acids (LiAlH4 in dry ether, room temperature)
why are alcohols weaker acids than water?
- alkyl groups (on alcohol) are inductively electron-donating and will intensify the negative charge on the alkoxide ion, destablising it
- the alkoxide ion hence has a greater tendency, compared to the hydroxide ion, to accept and proton and re-form the alcohol molecule
- so alcohols are less easily deprotonated -> lower extent of dissociationg compared to water -> lower Ka -> higher pKa
how do electron-withdrawing groups, like -NO2, -Cl & -COOH, affect the acidity of alcohols?
- the presence of electron-withdrawing groups stabilises the alkoxide ion formed by dispersing the negative charge on the alkoxide ion, stabilising it
- this promotes the ionisation of the alcohol -> greater extent of dissociation -> higher Ka -> lower pKa
how does the distance of the electron-withdrawing group from the -OH group affect the acidity of the alcohol?
- inductive effect decreases with increasing distance
- the nearer the electron-withdrawing group is to the negatively-charged oxygen in the alkoxide ion, the stronger the electron-withdrawing effect -> greater degree of dispersion of the negative charge
- ionisation of the alcohol is promoted -> greater degree of dissociation -> higher Ka -> lower pKa
how do electron-donating groups, like -CH3, affect the acidity of alcohols?
- electron-donating groups intensify the negative charge on the alkoxide ion, destabilising the alkoxide ion
- the alkoxide ion is hence more likely to re-accept a proton to reform the alcohol -> lower extent of dissociation -> lower Ka -> higher pKa
why are phenols more acidic than alcohols & water?
- in the phenoxide, the negative charge on the negatively charged oxygen can delocalise into the benzene ring to some extent -> the phenoxide ion is stabilised to a larger extent than the alkoxide ion -> the phenoxide anion is less likely to accept a proton to re-form phenol -> greater extent of dissociation than alcohol -> stronger acid than alcohol -> higher Ka than alcohol -> lower pKa than alcohol
how does the presence of electron-withdrawing substituents affect the acidity of phenols?
- the presence of electron-withdrawing groups on the benzene ring withdraws more electron density from the negatively-charged oxygen -> disperse negative charge, stabilising the phenoxide ion & promoting its ionisation -> greater extent of dissociation -> higher Ka -> lower pKa
how does the presence of electron-donating substituents affect the acidity of phenols?
- the presence of electron-donating groups on the benzene ring reduces delocalisation of the negative charge on the oxygen atom into the ring (intensifies negative charge on phenoxide ion) -> destabilises phenoxide ion -> more likely to accepta proton to re-form the phenol -> lesser extent of dissociation -> decreased acid strength -> lower Ka -> higher pKa
acid-metal reaction
what are the reagents, conditions & observations for reaction of alcohols with group 1 metals (e.g. Na)?
reagents & conditions: Na (s), room temperature
observation: slow effervescence of hydrogen gas
acid-metal reaction
what are the reagents, conditions & observations for reaction of phenols with group 1 metals (e.g. Na)?
reagents & conditions: Na (s), room temperature
observation: rapid effervescence of hydrogen gas
note: RAPID effervescence for phenol reaction with Na, SLOW effervescence for alcohol reaction with Na
acid-base reaction
what are the reagents, conditions & observations for reaction of phenols with bases (e.g. NaOH)?
reagents & conditions: NaOH (aq), room temperature
observation: cloudy mixture dissolves to form colourless homogeneous solution
phenols are NOT ACIDIC ENOUGH to react w/ carbonates to liberate CO2 (g)
alcohols (weaker acids than water) are not acidic enough (refer flashcard 7) to react with aqueous sodium hydroxide or carbonates to form the corresponding alkoxides
nucleophilic substitution
what are the reagents, conditions & observations for reaction of alcohols with phosphorus (V) chloride?
reagents & conditions: PCl5, room temperature
observation: dense white fumes of HCl produced
can be used as distinguishing test for presence of alcohol group (if -COOH is absent)
nucleophilic substitution
what are the reagents & conditions for reaction of alcohols with phosphorus trihalides (e.g. PCl3)?
reagents & conditions: PCl3, room temperature OR P, Br2/I2, heat
halogenoalkanes & phosphoric (III) acid is formed
PBr3 & PI3 are prepared in situ by heating P with Br2 & I2 respectively
nucleophilic substitution
what are the reagents, conditions & observations for reaction of a tertiary alcohol with hydrochloric acid?
reagents & conditions: conc. HCl, room temperature
observation: the solution turns cloudy
forms chloroalkane
what are the reagents, conditions & observations for reaction of a primary/secondary alcohol with hydrochloric acid?
reagents & conditions: conc. HCl, ZnCl2, heat
observation: solution turns cloudy
forms chloroalkane
the reaction of primary/secondary alcohols with conc. HCl is slower so anhydrous ZnCl2 is added as a catalyst and the mixture must be HEATED (vs for tertiary alcohol, no need heat & catalyst
nucleophilic substitution
what are the reagents & conditions for reaction of alcohols with hydrogen bromide?
reagents & conditions: conc. HBr, heat OR NaBr, conc. H2SO4, heat
forms bromoalkane
nucleophilic substitution
what are the reagents, conditinos & observations for reaction of alcohols with thionyl chloride (SOCl5)?
reagents & conditions: SOCl2, warm
observation: SO2 gas & white fumes of HCl produced
forms chloroalkane
can be used as distinguising test for alcohol group (if -COOH is absent)
gaseous by-products are easy to separate from the liquid product
why are phenols must less susceptible towards nucleophilic substitution?
- the lone pair of electrons on the O atom in phenol delocalises into the benzene ring resulting in partial double bond character in the C-O bond -> increases C-O bond strength in phenols compared to in alcohols -> C-O bond is more difficult to break
- the approach of a nucleophile to the rear side of the C-O bond is sterically hindered by the benzene ring. the pi electron cloud of the benzene ring also repels the lone pair of electrons on the approaching nucleophile
what are the reagents & conditions for dehydration (elimination) of alcohols?
reagents & conditions: heat, Al2O3/conc. H3PO4/excess conc. H2SO4
forms alkenes
ONLY alcohols with at least 1 H atom of carbon atom ADJACENT to the carbon atom bearing the -OH group can undergo dehydration
conc. H3PO4 is usually preferred as H2SO4 may act as an oxidising agent, forming unwanted side products
no reaction when using phenols instead of alcohols
