HS Chemistry - Essential Functional Groups

Alcohols

Overview of The Page

This page will cover:

• What are alcohols?
• How are alcohols named? What are the different types of alcohols?
• What reactions do alcohols participate in?

Alcohols are organic molecules that contain the OH^- ion $the hydroxyl group$. They can be classified into three categories:

• Primary alcohols are alcohols where the Carbon atom that the hydroxyl group is connected to is also bonded to two Hydrogen atoms and one other group. An example is shown below:

  

  The skeletal formula of this alcohol would be:

  

  
  

• Secondary alcohols are alcohols where the Carbon atom that the hydroxyl group is connected to is also bonded to one Hydrogen atoms and two other groups. An example is shown below:

  

  The skeletal formula of this alcohol would be:

  

  
  

• Tertiary alcohols are alcohols where the Carbon atom that the hydroxyl group is connected to is also bonded to three other groups. An example is shown below:

  

  The skeletal formula of this alcohol would be:

  

To form the name for an alcohol, we use the same naming convention as we do for other organic molecules. The suffix ending for alcohols is "-ol", but the way it's added is different. Instead of removing the entire suffix, we take the suffix of the hydrocarbon and add "-ol" to the end. Thus, if we have the following molecule:

Where there is both an alkene functional group and a hydroxyl group, we take the alkene suffix $"\-ene"$ and add "-ol" to the end after removing the "e" at the end $giving us "\-enol"$. Thus, the molecule shown above is 1,1-butenol, or but-1-en-1-ol.

Following that same logic, this:

Is 1-butanol, or butan-1-ol.

Additionally, recall how when writing alkenes we want to note the position number of the double bond. Similarly, when writing alcohols, we want to use the same convention to note the position number of the hydroxyl group.

However, to return the discussion to the different classifications of alcohols - how can we, given an alcohol compound, test and determine what type of alcohol it is?

We can mix the alcohol with a solution of acidified potassium dichromate $K~2~Cr~2~O~7~$, which has an orange color, and then warm the solution. If the solution turns green, then the alcohol in question is either a primary or secondary alcohol, and it has been oxidized. If it remains orange, then the alcohol in question is a tertiary alcohol, and it has not been oxidized. This is the oxidation reaction for alcohols.

Since alcohols contain Oxygen atoms, which have a higher electronegativity $3.5$ than Carbon atoms $2.5$, an alcohol with a certain amount of Carbon atoms in its main chain tends to have a higher boiling point than an alkane or alkene with the same amount of Carbon atoms in its main chain.

Additionally, since the bond between Oxygen atom and the Hydrogen atom in a hydroxyl group is polar $the electronegativity difference between the two atoms is greater than 0.5$, a dissolved alcohol can donate Hydrogen ions. For this reason, alcohols are considered weakly acidic.

Given this information, we can determine that primary alcohols are the strongest acid of the three types of alcohols, and tertiary alcohols are the weakest acid. The reason for this is because tertiary alcohols have more electrons being pushed onto their hydroxyl group from the other groups that are attached to the Carbon atom to which the hydroxyl group is bonded, and this makes the Oxygen atom more electronegative, which makes it hold on to the Hydrogen atom more strongly. It thus donates the Hydrogen less readily, making it the weakest acid. Conversely, primary alcohols, with the fewest number of nearby groups, have the least electronegative Oxygen atom of the three types of alcohols. Thus, they donate the Hydrogen more readily, making it the strongest acid of the three types of alcohols.

Alcohol Reactions

There are several reactions alcohols can undergo:

• Combustion:

  • In a combustion reaction, the alcohols react with O₂ molecules in the air to produce CO₂ and H₂O. This is what allows alcohols to be used as biofuels in place of fossil fuels.

• Substitution With A Halogen/Halide:

  • A halogen or Hydrogen halide can react with an alcohol to form a halogenoalkane. As the Oxygen atom is more electronegative than the Carbon atom, it leaves the Carbon atom with a slight positive charge. A nucleophilic halogen or Hydrogen halide can then attack that bond, removing the hydroxyl group. If a Hydrogen halide, substitutes with the group, the Hydrogen that gets left behind reacts with the newly loose hydroxide ion to form H₂O.
  • If a sulfur halide oxide is used instead of a Hydrogen halide, then the substitution still occurs, but instead of ending up with H₂O, we end up with a Hydrogen halide and sulfur dioxide $SO~2~$.
  • If a phosphorus halide is used instead of a Hydrogen halide, then the substitution will still occur, but instead of ending up with H₂O, we end up with a Hydrogen halide and phosphoryl chloride $POCl~3~$.

• Substitution Reactions With Metals:

  • In this reaction, the metal simply substitutes for the Hydrogen atom that is part of the hydroxyl group $i.e. the Hydrogen atom that is bonded to the Oxygen atom$, and Hydrogen gas is released. Any metal can be used for this, although usually Group 1 metals are used $and sometimes Group 2 metals$.

• Dehydration:

  • In Alkenes, it was mentioned how adding water to an alkene would produce an alcohol. The dehydration reaction of an alcohol reverses that process. By passing the alcohol at high temperatures over an Al₂O₃ catalyst or heating it to high temperatures with concentrated H₂SO₄, the alcohol is broken down into an alkene and H₂O.