Alcohols, Ethers, and Epoxides - Synthesis | Organic Chemistry 1

The alcohols, ethers, and epoxides are studied in this chapter: properties and naming of alcohols, synthesis of alcohols (SN reaction, reduction of carbonyls, organometallics), Williamson ether synthesis, intramolecular Williamson epoxide synthesis

Properties of Alcohols

Alcohol:

A compound having the general structure ROH. An alcohol contains a hydroxy group (OH group) bonded to an sp3 hybridized carbon atom 

 

General properties:
 

  • Alcohols exhibit molecular polarity due to the electronegativity of oxygen
  • Alcohols have a hydrophobic part (the carbon chain) and a hydrophilic part (the OH group):
    - alcohols with less than 6 carbons are soluble in water
    - alcohols with 6 or more carbons are insoluble in water because the nonpolar alkyl portion is too large to be dissolved H2O
    - alcohols of any size are soluble in organic solvents

 

  • The hydroxy functional groups form hydrogen bonds with other alcohol molecules ⇒ alcohols have higher boiling and melting points than the corresponding alkanes

 

  • Alcohols are amphoteric: they can be deprotonated by strong bases or protonated by strong acids

 

Nomenclature of Alcohols

How to name an alcohol:
 

The -ane ending of the corresponding alkane is replaced by -anol

  1. Find the longest carbon chain containing the carbon bonded to the OH group

  2. Number the carbon chain starting from the end closest to the OH group (if equal, start with the end closest to the first substituent)

  3. Write the number corresponding to the location of the OH group before the parent name
    Use the suffixes diol, triol ... if there is more than one OH group
    When an OH group is attached to a ring, the ring is numbered starting with the OH group (the "1" is usually omitted from the name)

  4. Identify the substituents and assign locators

  5. Assemble the substituents alphabetically as prefixes

 

Preparation of Alcohols by SN Reactions

 


Mechanism:

SN2 reactions using HO- as nucleophile. As in all SN2, the reaction works best for CH3X and primary alkyl halides
 

 

Synthesis of Alcohols by Reduction of Carbonyls

Carbonyl groups:  

Functional groups composed of a C=O bond. They are formed by oxidation of alcohols using Na2Cr2O7 or PCC

​​​​Carbon atom is less electronegative than oxygen: it is poor in electrons and will react as an electrophile. On the contrary, oxygen of a carbonyl group is nucleophilic

 

Reduction of carbonyls:



Lithium aluminium hydride (LiAlH4) and sodium borohydride (NaBH4) contain a polar metal-hydrogen bond which is a source of nucleophilic hydride H-

Mechanism:

  1. Nucleophilic attack of H- - Formation of a carbon-hydrogen bond


     
  2. Protonation of the alkoxide

 

 

​​​Oxydation and reduction of alcohols:

Primary alcohol:

Secondary alcohol:

 

Synthesis of Alcohols with Organometallics

Organometallics:

Chemical compounds containing at least one chemical bond between a carbon and a metal

3 common types of organometallics R-M (with R = alkyl chain):

  • Alkyl lithium RLi
  • Organomagnesium or Grignard reagent RMgBr
  • Organocuprates or Gilman reagent R2CuLi

 

Reaction with aldehydes and ketones:



Mechanism: addition of R'- and H+ to the carbonyl
Addition to aldehydes forms a secondary alcohol while addition to ketones forms a tertiary alcohol

  1. Nucleophilic attack of R'- - Formation of a carbon-carbon bond


     
  2. Protonation of the alkoxide


Other common reactions with organometallics:
 

  • Strong reaction with water:
  • Carbon-carbon coupling reaction with Gilman reagent:

Synthesis of Ethers

Ether:

A class of organic compounds that contain an oxygen atom connected to two alkyl or aryl groups

 

Williamson ether synthesis:



Mechanism: SN2 reaction
The reaction works best with CH3X and primary alkyl halides

Synthesis of Epoxides

Epoxide:

A cyclic ether having the oxygen atom as part of a three-membered ring. The strain of the three-membered ring makes an epoxide much more reactive than a typical acyclic ether

 

Intramolecular Williamson epoxide synthesis:



Mechanism:

  1. Deprotonation of the hydroxy group - Formation of an alkoxide


     
  2. Intramolecular SN2 reaction

Check your knowledge about this Chapter

An alcohol is a compound with the general structure ROH. It contains a hydroxy group (OH group) bonded to a sp3 hybridized carbon atom

  • Alcohols exhibit molecular polarity due to the electronegativity of oxygen
  • Alcohols have a hydrophobic part (the carbon chain) and a hydrophilic part (the OH group):
    - alcohols with less than 6 carbons are soluble in water
    - alcohols with 6 or more carbons are insoluble in water because the non-polar alkyl part is too large to be dissolved in H2O
    - alcohols of any size are soluble in organic solvents
  • The hydroxy functional groups form hydrogen bonds with other alcohol molecules ⇒ alcohols have higher boiling and melting points than the corresponding alkanes
  • Alcohols are amphoteric: they can be deprotonated by strong bases or protonated by strong acids

The -ane ending of the corresponding alkane is replaced by -anol

  1. Find the longest carbon chain containing the carbon bonded to the OH group
  2. Number the carbon chain starting from the end closest to the OH group (if equal, start with the end closest to the first substituent)
  3. Write the number corresponding to the location of the OH group before the parent name
    Use the suffixes diol, triol ... if there is more than one OH group
    When an OH group is attached to a ring, the ring is numbered starting with the OH group (the "1" is usually omitted from the name)
  4. Identify substituents and assign locators
  5. Assemble the substituents alphabetically as prefixe

Alcohols can be prepared by SN2 reactions with HO- as nucleophile, by reduction of carbonyls (mainly aldehydes and ketones) or by reaction of carbonyls with organometallics. Alcohols can also be prepared by the hydration of alkenes, as we will see in the chapter on alkenes

Alkyl halides can be converted to alcohols by an SN2 reactions using HO- as strong nucleophile. As in all SN2, the reaction works best with CH3X and primary alkyl halides

Aldehydes and ketones can undergo a reduction process for the formation of a primary or secondary alcohol with sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4). NaBH4 and LiAlH4 contain a polar metal-hydrogen bond that is a source of nucleophilic hydride H-

The carbon-oxygen double bond is highly polar, and the slightly positive carbon atom is attacked by the hydride ion of sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4). The second step is the protonation of the alkoxide in the presence of water

Primary alcohols can be oxidized to form aldehydes and carboxylic acids depending on the reaction conditions; secondary alcohols can be oxidized to ketones

Primary alcohols can be oxidized to aldehydes using pyridinium chlorochromate (PCC) or to carboxylic acids using sodium dichromate (Na2Cr2O7) under acidic conditions. In the case of the formation of carboxylic acids, the alcohol is first oxidized to an aldehyde, which is then oxidized further to the acid

Organometallics are chemical compounds containing at least one chemical bond between a carbon and a metal

The 3 common types of organometallics R-M (with R = alkyl chain) are:

  • Alkyl lithium RLi
  • Organomagnesium or Grignard reagent RMgBr
  • Organocuprates or Gilman reagent R2CuLi

Organometallics are very powerful nucleophiles that attack at the carbonyl group to give alcohols, forming a new C-C bond. It is one of the most important ways to make carbon-carbon bonds. Addition to aldehydes forms a secondary alcohol, while addition to ketones forms a tertiary alcohol

Ethers are a class of organic compounds containing an oxygen atom connected to two alkyl or aryl groups and having the formula R-O-R’

One way to make ethers is to use the Williamson ether synthesis, an SN2 reaction in which an alkoxide ion is a nucleophile that displaces a halide ion from an alkyl halide to give an ether

The Williamson ether synthesis works best with CH3X and primary alkyl halides

An epoxide is a cyclic ether having the oxygen atom as part of a three-membered ring. The strain of the three-membered ring makes an epoxide much more reactive than a typical acyclic ether

Epoxides can be synthesized from a halohydrin via an intramolecular Williamson epoxide synthesis. In this two-step reaction sequence, removal of the proton from the hydroxy group with a base forms an alkoxide which then reacts as a nucleophile in an intramolecular SN2 reaction to form the epoxide