Alcohols

Solo disponible en BuenasTareas
  • Páginas : 6 (1348 palabras )
  • Descarga(s) : 0
  • Publicado : 12 de diciembre de 2011
Leer documento completo
Vista previa del texto
ALCOHOLS

Alcohols Properties Physical
No gaseous alcohols are known. The lower members of the homologous series of aliphatic alcohols (containing C1 to C10) are clear colourless liquids at room temperature. They have varying solubility in water, the higher alcohols being less soluble. The alcohols higher than C12 are solids and are insoluble in water.
Methanol, ethanol and propanol aremiscible with water. The alcohols are miscible in all proportions with most organic liquids. As we pass up the series, the specific gravity increases.
The boiling points of the straight chain alcohols increase as the number of carbon atoms in the molecule increase. For a given molecular weight, there is a decrease in the boiling point when branching of carbon atoms occurs. Thus, the primary alcoholsboil at a higher temperature than the secondary alcohols of the same molecular weight, and similarly, secondary alcohols have higher boiling points than the tertiary alcohols. The boiling points are much higher than is to be expected from their molecular weights. For example, the boiling point of ethanol, 78 degC, can be explained by the attraction of ethanol molecules by means of hydrogen bondsto form extended groups of molecules.
Hydrogen bonds can arise in ethanol because the area around the oxygen atom is relatively rich in electrons and can attract hydroxyl hydrogen from a neighboring ethanol molecule. These intermolecular bonds are considered to be intermediate in strength between weak Van der Waals' forces and the strong forces between ions. The extra energy required to break thehydrogen bonds leads to an increase in boiling point .
The alcohols react with sodium and potassium with the evolution of hydrogen.

http://www.ucc.ie/academic/chem/dolchem/html/dict/alcohols.html

CHEMICAL PROPERTIES

Oxidising the different types of alcohols
The oxidising agent used in these reactions is normally a solution of sodium or potassium dichromate(VI) acidified with dilutesulphuric acid. If oxidation occurs, the orange solution containing the dichromate(VI) ions is reduced to a green solution containing chromium(III) ions.

The electron-half-equation for this reaction is

Secondary alcohols
Secondary alcohols are oxidised to ketones - and that's it. For example, if you heat the secondary alcohol propan-2-ol with sodium or potassium dichromate(VI) solutionacidified with dilute sulphuric acid, you get propanone formed.
Playing around with the reaction conditions makes no difference whatsoever to the product.
Using the simple version of the equation and showing the relationship between the structures:

If you look back at the second stage of the primary alcohol reaction, you will see that an oxygen "slotted in" between the carbon and the hydrogen inthe aldehyde group to produce the carboxylic acid. In this case, there is no such hydrogen - and the reaction has nowhere further to go.

Tertiary alcohols
Tertiary alcohols aren't oxidised by acidified sodium or potassium dichromate(VI) solution. There is no reaction whatsoever.
If you look at what is happening with primary and secondary alcohols, you will see that theoxidising agent is removing the hydrogen from the -OH group, and a hydrogen from the carbon atom attached to the -OH. Tertiary alcohols don't have a hydrogen atom attached to that carbon.
You need to be able to remove those two particular hydrogen atoms in order to set up the carbon-oxygen double bond.



Primary alcohols
Primary alcohols can be oxidised to either aldehydes orcarboxylic acids depending on the reaction conditions. In the case of the formation of carboxylic acids, the alcohol is first oxidised to an aldehyde which is then oxidised further to the acid.

Partial oxidation to aldehydes
You get an aldehyde if you use an excess of the alcohol, and distil off the aldehyde as soon as it forms. The excess of the alcohol means that there isn't enough oxidising...
tracking img