NOMENCALTURE OF ALCOHOLCommon Naming of alcohols:Number the-longest carbon chain so that the -OH group isattached to the carbon atom with the lowest possiblenumber.Name the parent compound by using the alkane name andreplacing the -e ending with an -ol ending.Indicate the position of the hydroxyl group with a numberin any alcohol containing three or more carbon atoms.
1. Aliphatic alcoholi. The parent alcohol - the longest C chain with OH group attached to it OH group is given the lowest number in the chain. prefix & suffix used if alkyl groups are exist more than one times
ii. When there are two or more –OH groups present, the name ends with diol, triol and so on. OH OH CH3CHCH2CHCH3 2,4-pentanediol OH CH3CCH2OH 1,2,2-propanetriol OH
2. Aromatic alcohol (phenols) In most cases, the name phenol is used as the parent’s name. OH OH NO2 CH3 4-methylphenols 3-nitrophenols(4-methyl hydroxybenzene)
Physical Properties of Alcohol:As the alkane-like alkyl group (hydrophobic @water hating) becomes larger, water solubilitydecrease. H O H hydrogen bond R OH
(i) Boiling point The boiling point of an alcohol is always much higher than that of the alkane with the same number of carbon atoms. Intermolecular foces: Between alkanes, the presence of van der Waals forces. Between alcohols, the presence of hydrogen bonding Hydrogen bonding are much stronger than VdW forces and therefore it takes more energy to separate alcohol molecules than it does to separate alkane molecules. Therefore, boiling point of alcohols is higher than alkanes.
• The boiling points of the alcohols increase as the number of carbon atoms increases. – Alcohols with lower number of carbons has smaller size compared to alcohols with higher number of carbons. – Alcohols with smaller size will have weaker van der Waals forces instead of hydrogen bonding that is formed between OH group. – Therefore, alcohols with lower number of carbons have lower boiling points.
Volatility• But they also cause alcohols to have a lower volatility than alkanes of a similar molecular mass Solubility• Alcohols dissolve in water because hydrogen bonds form between the polar –O-H groups of the alcohols and water molecules• The first three members of the homologous series are soluble in water• Solubility decreases as the chain length increases; the larger part of the alcohol molecule is made up of a non- polar hydrocarbon chain. Also the hydrocarbon part of the chain doesn’t form hydrogen bonds with water
Chemical properties of Alcohols• As an acid and as a base- Alcohol can donate or accept a proton1. Reaction with Sodium (alcohol as acid)2. Dehydration of alcohols to yield alkenes (alcohol as basic)
1.Reaction with Sodium (alcohol as acid) As an acid R-OH + H2O RO- + H3O+• Alcohol reacts with Sodium to form sodium alkoxide and hydrogen gas.• This reaction shows the acidic properties of alcohol. eg: RO-H + Na RO-Na+ + 1/2H2 CH3CH2OH + Na CH3CH2O-Na+ + 1/2H2 (sodium alkoxide)
2.Dehydration of alcohols to yield alkenes As a baseR-CH2-OH + HA RCH2-OH2 + A-eg:CH3-CH2-OH + H2SO4 conc CH2=CH2 + H2O Refer to the Saytzeff Rule to predict major alkene product.
Saytzeff Rule• major product is the more stable alkene.• Which is the more stable alkene? – the most number of alkyl groups attached to the C = C
Preparation of Alcohol:(a) Fermentation of Carbohydrates : by yeast eg: C6H12O6 2CH3CH2OH + 2CO2 sugar ethanol This process still widely used to prepare alcohol
(b) Hydration of Alkenes- Alkenes was added by H2O and H2SO4 / H3PO4. This reaction follows Markovnikov’s Rule eg: R-C=CH2 + H2O / H+ R-C-CH3 Alkene Alcohol
Markovnikov Rule:• In addition of electrophilic to an alkene, the more positives reagent adds to the carbon double bond that have largest number of hydrogen bonded to it.
(c) Hydrolysis of Haloalkanes- Alkyl Halide react with strong bases (NaOH) to produce alcohols and Sodium Halides (NaX) eg: R-X + OH- R-OH + X- H2O/reflux CH3-Br + NaOH CH3-OH + NaBr
(d) Addition of Grignard Reagents to Carbonyl Compounds- We can use aryl, alkyl and vinylic halides react with magnesium in ether solution to generate Grignard reagents, RMgX. ether- R-X + Mg RMgX where R = alkyl/aryl/vinyl
• These Grignard reagents react with carbonyl compounds to yield alcohol. hydrolysis / H3O+ RMgX + RCHO R2CHOH Grignard alcohol reagent
Mg / etherCH3CH2 X CH3CH2 MgBr O H2O / H+ H C CH3 carbonyl OH CH3CH2CCH3 H
(e) Reaction with carboxylic acid to form an ester (esterification)general equation: H+RCOOH + R’OH RCOOR’ + H2Oeg:CH3COOH + CH3CH2OH CH3COOCH2CH3 + H2Ocarboxylic acid alcohol ester water(etanoic acid) (ethanol)
(f) Hydrogenation of Alcohol• Dehydrate with conc. H2SO4,• then add H2 - Hydrogenation OH H2SO4 H2CH3CHCH3 CH2 CHCH3 CH3CH2CH3 Pt alcohol alkene alkane
(g) Reaction with Hydrogen Halides, Phosphorus Halides (PX3 /PX5) and Thionyl Chloride (SOCl2)- When alcohol react with a hydrogen halide, a substitution takes place producing alkyl halide and water. General reaction: (i) R - OH + H X R - X + H2O (ii) 3ROH + PX3 3RX + H3PO3 (iii) ROH + PCl5 RCl + POCl3 + HCl (iv) ROH + SOCl2 RCl + SO2 + HCl
(h) Formation of Tosylate• Typical acids used for alcohol dehydration are H2SO4 or p-toluenesulfonic acid (TsOH).• More substituted alcohols dehydrate more easily, giving rise to the following order of reactivity.
Tosylate: A Good Leaving Group• Alcohols are converted to tosylates by treatment with p-toluenesulfonyl chloride (TsCl) in the presence of pyridine.• This process converts a poor leaving group (¯OH) into a good one (¯OTs).• Tosylate is a good leaving group because its conjugate acid, p-toluenesulfonic acid (CH3C6H4SO3H, TsOH) is a strong acid (pKa = -7).
Tosylate: A Good Leaving Group• (S)-2-Butanol is converted to its tosylate with retention of configuration at the stereogenic center. Thus, the C—O bond of the alcohol is not broken when tosylate is formed.
Tosylate: A Good Leaving Group• Because alkyl tosylates have good leaving groups, they undergo both nucleophilic substitution and elimination, exactly as alkyl halides do.• Generally, alkyl tosylates are treated with strong nucleophiles and bases, so the mechanism of substitution is SN2, and the mechanism of elimination is E2.
Tosylate: A Good Leaving Group • Because substitution occurs via an SN2 mechanism, inversion of configuration results when the leaving group is bonded to a stereogenic center. • We now have another two-step method to convert an alcohol to a substitution product: reaction of an alcohol with TsCl and pyridine to form a tosylate (step 1), followed by nucleophilic attack on the tosylate (step 2). 34
Tosylate: A Good Leaving Group• Step 1, formation of the tosylate, proceeds with retention of configuration at a stereogenic center.• Step 2 is an SN2 reaction, so it proceeds with inversion of configuration because the nucleophile attacks from the backside.• Overall there is a net inversion of configuration at a stereogenic center. Example: 35