SCH 4U - Alcohols (R-OH) & Ethers (R-O-R)

Alcohols (R-OH) 
All members of the alcohol family will contain the hydroxyl (-OH) functional group.

Some common alcohols are ethanol (in spirits, like wine and beer), cholesterol (a type of fat produced by the bodies of animals) and retinol (vitamin A). 

Naming Alcohols 
The ending ‘-ol’ is added to the name of the parent compound (the longest carbon chain) and a numbering system is used to locate the –OH group when necessary. 

We use the same rules as usual to name alcohols.  1.  Find the longest chain. 2.  Number the main chain to give all substituents (as well as the hydroxyl group) the lowest possible numbers. 3.  List substituents in alphabetical order. 4.  Finish with the main chain, which includes info about the alcohol.  For instance, the third compound is numbered from right to left.  This places methyl groups at carbons 2 and 4 (2,4-dimethyl). The main chain is six carbons long (hex); the carbons in the main chain are singly bonded (an); the hydroxyl group is found at carbon 4 (-4-) and the family is alcohol (ol).

1°, 2° & 3° Alcohols

Alcohols are classified according to the type of carbon atom to which the –OH group is attached.  If this C atom has 1, 2 or 3 alkyl groups attached to it, in addition to the –OH group, the resulting alcohol is said to be primary, secondary or tertiary respectively. In order, the above three compounds are primary, secondary and tertiary.

Polyalcohols

Alcohols that contain more than one –OH group, use prefixes like ‘-diol’ and ‘-triol’ to indicate the number of –OH groups on the molecule.

Sometimes a compound has both a common name (or two) and an IUPAC name.  Notice that when there is more than one OH group, the 'e' at the end of the main chain name comes back into play.  For instance, a two carbon chain with one OH group would be named ethanol.  However, if another OH group is added to the other carbon in this two carbon chain, the name is ethane-1,2-diol (as above).  The reason the 'e' shows back up is because of the old way of naming.  In the old method, antifreeze would have the IUPAC name 1,2-ethanediol (hello, 'e').  It was written like this because it rolls off the tongue better than 1,2-ethandiol (no 'e').  So, even though we use the new system, we maintain some of the nostalgia of the old system.

Cyclic Alcohols

If the –OH group is attached to a ring, it is referred to as a cyclic alcohol.  Many large molecules are known by their common names; menthol and cholesterol to name a pair.  There are also aromatic alcohols, the simplest of which is phenol or hydroxybenzene. 

On the left: cyclopentanol.  It is a cyclopentane with a hydroxyl group attached.  There is no need to locate the OH group because all the carbons in the ring are equivalent.            On the right: phenol or hydroxybenzene.  When benzene is involved in any compound, you are guaranteed to see 'benz' or 'phen' in the name.  So, benzene + OH group = phenol.  It is also IUPAC appropriate to represent the OH group with the name hydroxy (but, only in this case, in high school, we do not use 'hydroxy' at any time other than when attached to benzene).  So, OH group attached to benzene = hydroxybenzene.

Properties of Alcohols 
The properties of the alcohols are based on the nature of the hydroxyl group:

-OH 

• polar and can H-bond
• expect to have higher melting and boiling points for alcohols than their parent alkanes (for instance, look at the major difference in these two bp:  ethane ↝ -89°C; ethanol ↝ 78°C)
• water solubility of alcohols is better than that of the analogous hydrocarbons, due to the H-bonding ability

Reactions Involving Alcohols
(I)  Preparing Alcohols:  Hydration Reactions 
Recall that alkenes readily undergo addition (hydration) reactions.  Starting with an alkene and adding HOH, will introduce the –OH functional group, producing an alcohol.  Just don't forget to apply Markovnikov's rule when necessary.

NameIt!  What are the names of the organic molecules in this reaction?  See the end of the lesson for the answer.

Ethanol can also be prepared by fermentation of sugars, using a yeast culture, in the absence of oxygen: 

C₆H₁₂O₆(s)  →   2CO₂(g)   +   2C₂H₅OH(l)

Methanol, which cannot be produced by an addition reaction, is produced by combining carbon monoxide and hydrogen at a high temperature and pressure in the presence of a catalyst:

CO(g)   +   H₂(g)   →   CH₃OH(l) 

(II)  From Alcohols to Alkenes:  Elimination Reactions 

The addition reaction discussed previously can be carried out in reverse.  Since the elimination reaction results in water removal, it is also called a dehydration reaction. 

The sulfuric acid catalyst causes the removal of water from the alkane.  The OH group and a H on the C adjacent to the hydroxyl-bearing carbon are removed.  The two carbons that lost the OH and H respectively, work together to form a second bond, which results in a double bond.  The OH and H that were removed, join together to form a water molecule as the other product.            NameIt!  What are the names of the organic molecules in this reaction?  See below for the answer.

Ethers (R-O-R′) 
Formerly used as an anaesthetic (diethyl ether).  Prior to the discovery and use of ether in the mid-1800s, surgeries were performed without anaesthesia warning the images at this link are...yikes).  The use of ether revolutionized medicine.

Ethers can either be symmetrical (R-O-R) or asymmetrical (R-O-R′), depending on whether the alkyl groups are the same or different. 

Naming Ethers
Ethers can be named by stating, in alphabetical order, the names of the hydrocarbons followed by ‘ether’ (if the two sides are the same, the prefix ‘di-‘ is used). 

**Note: There are two accepted methods for naming an ether.  I am only going to show you one method - if you wish to know the other, message me.**            Starting at the Cs attached directly to the -O-, number outward in either direction along the longest chains (on the left there is an unsubstituted three C chain and on the right, there is a branched six C chain).  Name both sides as substituents (on the left, we have propyl and on the right we have 3-ethylhexyl).  Place the two in alphabetical order with a space between them: 3-ethylhexyl propyl.  Then add a space and "ether" to give:  3-ethylhexyl propyl ether.            If both substituents are the same alkyl group, the prefix "di" is employed.  For instance, diethyl ether has two ethyl groups attached to the central oxygen.

Properties of Ethers 

The properties of the ethers are based on the characteristics imparted by the -O- within the hydrocarbon chain:

-O-

•  C-O-C bonds are polar, but cannot H-bond
• expect to have higher mp, bp and water solubility than analogous hydrocarbons (due to polarity)
• expect to have lower mp, bp and water solubility than analogous alcohols (due to loss of H-bond ability)

 

 

Preparing Ethers from Alcohols:  Condensation Reactions 

Ethers are formed by the reaction of two alcohol molecules, which eliminates a water molecule. 

The sulfuric acid catalyst removes H from the hydroxyl group of one alcohol (leaving CH₃CH₂O-) and the OH group from the other alcohol (leaving -CH₂CH₂CH₂CH₃).  This allows the two pieces to link together CH₃CH₂O─CH₂CH₂CH₂CH₃ and form an ether.  The eliminated H and OH combine to form the other product, HOH.            NameIt! What are the names of the organic molecules in this reaction?  See the end of the lesson for the answer.

NameIts! Answers: 
cis-but-2-ene, butan-2-ol
propan-2-ol, propene
ethanol, butan-1-ol, butyl ethyl ether

Homework #21-35