alcohols, phenols, and ethers Flashcards
(42 cards)
list the ways to prepare alcohols
1) from alkenes
a) By acid catalysed hydration
b) Hydroboration-oxidation
2) from carbonyl compounds
a) by reduction of aldehydes and ketones
b) by reduction of carboxylic acids and esters
3) from grignard reagents
explain acid catalysed hydration and its mechanism
Alkenes react with water in the presence of acid as catalyst to form alcohols in the presence of acid as catalyst to form alcohols. They follow markovnikovs rule in the case of unsymmetrical alkenes.
1) protonation of alkene by electrophilic attack of H3O+ to form carbocation
2) nucleophilic attack by water on carbocation
3) deprotonation to form alcohol
explain hydroboration
Diborane (BH3) reacts with alkenes to give trialkyl boranes as addition product which is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide. (FOLLOWS ANTIMARKOVNIKOV RULE AND GIVES EXCELLENT YIELD OF ALCOHOLS)
how to prepare alcohols by reduction of aldehydes and ketones
- by adding hydrogen in presence of catalysts which are finely divided metal such as palladium, nickel, platinum.
- also prepared by treating with sodium borohydride (NaBH4) and lithium aluminium hydride (LiAlH4).
- Aldehydes yield primary alcohols and ketones yield secondary alcohols
how to prepare from carboxylic acids and esters
- by lithium aluminium hydride which gives excellent yields
- commercially, by converting carboxylic acids to esters followed by catalytic hydrogenation
why lithium aluminium hydride not used?
very expensive, hence used for preparing special chemicals only
when do alcohols react as nucleophiles and electrophiles
Bond between O-H i s broken when they act nucleophilic and bond between C-O is broken when they act electrophilic
order in which acid strenght decreases
primary>secondary>tertiary. Electron donating groups increase the electron density on oxygen tending to decrease the polarity of OH bond therefore, acid strenght decreases.
why are phenols more acidic than alcohols
1) the phenoxide ion formed is stabilized due to resonance effect and the electron density decreases on oxygen due to the higher electronegativity of the sp2 hybridised carbon atom to which the OH group is attached, this increases the polarity of OH group and thus, acid strength increases.
2) in alkoxide ion the negative sharge is localised on the oxygen whereas in ohenoxide, the charge is delocalised, making phenoxide more stable. (DRAW RESONANCE STRUCTURES OF PHENOXIDE).
3) the resonance structures of phenol molecule has charge separation due to which phenol molecule is less stable than phenoxide ion.
why is cresol less acidic than o nitrophenol
cresol contains a methyl group which is an eelctron donating group thus increasing the electron dernsity on the O atom which increases the charge on the O atom hence decreasing the tendency to lose H atom whereas the nitrite group is electron withdrawing hence it decreases electron density on O atom and thus tendeny to lose H atom increases.
define acetylation and give one examplet
the introduction of acetyl group in alcohols or phenols is known as acetylation. Acetylation of salicylic acid gives aspirin.
why is reaction of alcohol with acid chloride carried in the presence of pyridine?
reaction of alcohol with acid chloride is carried out in the presence of a base (pyridine) which neutralizes the HCl formed thus shifting, the equilibrium towards the right hand side.
what is the reaction of alcohol with carboxylic acid carried out in the presence of?
it is carried out with the presence if a small amount of concentrated H2SO4. The reaction is reversible and thus, water is removed from the reasion as soon as it is formed which shifts the reaction in forward direction.
write equations of esterification with carboxylic acids, acid anhydrides, and acid chlorides
not telling you
other name of aspirin
acetylsalicylic acid
what is lucas test
The difference in reactivity of three classes of alcohols with HCl distinguishes them from one another using lucas reagent known as lucas test.
- Alcohols are soluble in Lucas reagent (conc. HCl and ZnCl2) whereas their halides are immiscible and produce turbidity in the solution.
- In case of primary alcohols, they do not produce turbidity at room temperature whereas, tertiary alcohols immediately produce turbidity as they form the halides easily.
what is alumina
Al2O3
dehydration
Alcohols undergo dehydration (removal of a molecule of water) to form alkenes on treating with a protic acid e.g., concentrated H2SO4 or H3PO4, or catalysts such as anhydrous zinc chloride or alumina.
order of relative ease of dehydration and conditions for ethanol, isopropyl alcohol, and tert-butyl alcohol
3°>2°>1°
-conditions for ethanol: conc.H2SO4 in 443 K
-isopropyl alcohol: 85% H3PO4, 440 K
-tert butyl alcohol: 20% H3PO4, 358 K
mechanism of dehydration
1) formation of protonated alcohol(oxonium ion) (fast)
2) formation of carbocation: it is the slowest step and hence, the rate determining step of the reaction.
3) deprotonation to form ethene
* the acid used in step 1 is realeased in step 3
list all reagents used for oxidation(dehydrogenation) of primary, secondary, tertiary alcohols
- 1°: better reagent for primary is PCC converting them directly to aldehydes in good yield.
- 2° alcohols are oxidised to ketones by chromic anhydride (CrO3).
- 3° alcohols do not undergo oxidation reactions at such conditions but under stronger reaction conditions such as strong oxidising agents like KMnO4 and at elevated temperatures, cleavage of various C-C bonds take place and a mixture of carboxylic acids with lesser number of carbon atoms is formed.
what is PCC
pyridinium chlorochromate, a complex chromium trioxide with pyridine and HCl
under oxidation what do primary and ssecondary alcohols form
aldehydes and ketones respectively
explain reactions of alcohols with copper
1°: when vapours of a primary alcohol are passed over heated copper at 573 K, dehydrogenation takes place and an ALDEHYDE is formed.
2°: when vapours of a secondary alcohol are pased over heated copper at 573 K, dehydrogenation takes place and a KETONE is formed.
3°: when vapours of a tertiary alochol are passed over heated copper at 573 K, DEHYDRATION takes place to form an ALKENE.
