Structure of Simpler Organic Compounds. 95 



placement of the hydrogen atom by an alkyl group are 

 known generally as esters. 



The derivatives of the halogen acids have been already 

 discussed (p. 62 and seq.). These have been regarded as 

 halogen substitution products of hydrocarbons, and some of 

 them can be produced by the method of direct substitu- 

 tion. The product, C 2 H 5 Br, for example, may therefore be 

 designated as monobrom ethane or ethyl bromide. Accord- 

 ing to the first title it would be regarded as a substitu- 

 tion product of ethane ; and according to the second an 

 ethyl ester of hydrobromic acid. The esters are more 

 generally produced from the alcohols by treatment of 

 these substances with the acids in the presence of a 

 dehydrating agent to remove the water 

 E-OH + HC1 RC1+H 2 



K-OH + R'-COOH ;n R' CO OR + H 2 

 The dehydrating agent is employed to remove the water 

 formed in the reaction. When acids and alcohols are 

 mixed, a reaction takes place. The change is a reversible 

 one, and the final composition of a mixture of an alcohol 

 and acid will depend upon the relative proportions of the 

 various substances in the mixture. When large amounts 

 of water are present, the principal reaction is in the 

 direction of the arrow pointing to the left that is, the 

 water tends to decompose the ester formed. If, however, 

 a dehydrating agent is present (such as concentrated 

 sulphuric acid), the water is removed from the sphere of 

 action, and the formation of the ester tends to proceed 

 towards completion that is, in the direction of the arrow 

 pointing to the right. This is, perhaps, the most general 

 method of preparing esters. The phenomenon of ester 

 formation, or " esterification," is governed by the general 

 laws of mass action, and need not be further discussed in 

 this place. 



