Low Molecular Weight Alcohols
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General methods of preparation of alcohols
➢ Hydration of Alkenes
Electrophilic hydration is the act of adding electrophilic hydrogen from a non-nucleophilic strong acid (a reusable catalyst, examples of which include sulfuric and phosphoric acid) and applying appropriate temperatures to break the alkene's double bond. After a carbocation is formed, water bonds with the carbocation to form a 1°, 2°, or 3° alcohol on the alkane.
The basic reaction under certain temperatures (given below) is the following:
Fig. 1 Electrophilic hydration reaction
- Primary Alcohol: when the temperature is less than 170℃, it mainly forms 1°alcohol
- Secondary Alcohol: when the temperature is less than 100℃, it mainly forms 2°alcohol
- Tertiary Alcohol: when the temperature is less than 25℃, it mainly forms 3°alcohol
➢ Hydroboration of Alkenes
Hydroboration of alkenes transforms alkenes into alcohols. It performs the net addition of water across an alkene.
Fig. 2 Hydroboration of alkenes reaction
Note that the oxygen is always attached at the less substituted carbon (anti-Markovnikoff). Furthermore, the stereochemistry is always syn (H and OH add to same side of the alkene).
➢ Reaction of Organometallic Compounds with Carbonyl Compounds
The nucleophilic carbon atoms of organometallic reagents react with the electrophilic carbon atoms of aldehydes, ketones, acyl halides, esters, and epoxides to build larger carbon chains. In the process, an alcohol is formed. These various reaction pathways are summarized below.
Fig. 3 Reaction of organometallic compounds with carbonyl compounds
➢ Reduction of Carbonyl Compounds with Metal Hydrides or Boranes
Carbonyl compounds are reduced by metal hydride reagents to give alcohols.
Fig 4. Reaction of carbonyl compounds with metal hydrides
The metal hydride reagents listed below are used commonly based on practical qualities such as availability and ease of handling:
| NaBH4 | LiAlH4 | (i-Bu)2AlH |
| NaBH3CN | LiBH4 | LiBHEt3 |
| BH3·L | Et3SiH | Red-Al |
| Ni(BH4)2 | NaBH(OAc)3 | Zn(BH4)2 |
| LiBH(s-Bu)3 | KBH(s-Bu)3 | Cp2ZrHCl |
| [(Ph3P)CuH]6 | (n-Bu)3SnH |
➢ Catalytic Hydrogenation of Carbonyl Compounds
The simplest large-scale procedure for reduction of aldehydes and ketones to alcohols is by catalytic hydrogenation:
Fig. 5 Catalytic hydrogenation of carbonyl compounds
The advantage over most other kinds of reduction is that usually the product can be obtained simply by filtration from the catalyst, then distillation. The common catalysts are nickel, palladium, copper chromite, or platinum activated with ferrous ion. Hydrogenation of aldehyde and ketone carbonyl groups is much slower than of carbon-carbon double bonds so more strenuous conditions are required.
➢ Meerwein–Ponndorf–Verley (MPV) Reduction of Aldehydes and Ketones
The Meerwein–Ponndorf–Verley (MPV) reduction in organic chemistry is the reduction of ketones and aldehydes to their corresponding alcohols utilizing catalysis in the presence of a sacrificial alcohol.
This reaction is particularly suited for the reduction of unsaturated aldehydes and ketones, compared to catalytic reductions with molecular hydrogen using noble metal catalysts. The MPV reaction has traditionally been carried out using homogeneous catalysts like aluminium or titanium alkoxides.
Fig. 6 Meerwein–Ponndorf–Verley (MPV) reduction
➢ Others
In addition to the synthesis methods listed above, alcohols can also be prepared by these methods: 1,2-glycols alkenes, hydrolysis of alkyl and allylic halides, hydrolysis of esters, aldol condensation, cleavage of ethers, etc.
Alfa Chemistry, as a leading supplier of low molecular weight alcohols, has the absolute advantages in custom synthesis. We are pleased to custom synthesize various of low molecular weight alcohols with best quality and provide full support to meet all your demands.
