Alkanols, Alkanoic Acids and Esters · 8 min read
Alkanols
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Introduction to Alkanols
Alkanols are a group of organic compounds where one or more hydrogen atoms in an alkane have been replaced by a hydroxyl group (-OH). In everyday life, we see them in methylated spirits, hand sanitizers, and even the fuel used in some specialized engines.
Simply put, alkanols are alkanes that carry the -OH group which gives them their special chemical properties.
Naming and Classification
To name an alkanol, we take the name of the parent alkane, remove the 'e' at the end, and add 'ol'. For example, Methane becomes Methanol, and Ethane becomes Ethanol. We can group them into three types based on the carbon atom holding the -OH group:
Primary (1o) Alkanols: The -OH group is attached to a carbon that is bonded to only one other carbon atom, like Ethanol (CH3 CH2 OH).
Secondary (2o) Alkanols: The -OH group is attached to a carbon bonded to two other carbon atoms, like Propan-2-ol.
Tertiary (3o) Alkanols: The -OH group is attached to a carbon bonded to three other carbon atoms, like 2-methylpropan-2-ol.
Physical Properties
1. The first few members are liquids at room temperature because of hydrogen bonding.
2. They are very soluble in water, but this solubility decreases as the carbon chain gets longer (like how thick palm oil doesn't mix with water as easily as methylated spirit).
3. They have higher boiling points than their corresponding alkanes because it takes more energy to break the hydrogen bonds between their molecules.
Chemical Properties
Alkanols undergo several reactions. One important reaction is Combustion, where they burn in oxygen to produce CO2 and H2O. This is why ethanol is used as a fuel in spirit lamps.
Another key reaction is Oxidation. When you use a strong oxidizing agent like acidified K2 Cr2 O7 (potassium heptaoxodichromate VI):
Primary Alkanols oxidize first to Alkanals, then to Alkanoic acids.
Secondary Alkanols oxidize to Alkanones (Ketones).
Tertiary Alkanols do not oxidize easily because they lack a hydrogen atom on the carbon holding the -OH group.
Worked Example: Molecular Formula
Question: Calculate the molecular formula of an alkanol with a relative molecular mass of 60. (C=12, H=1, O=16)
1. The general formula is Cn H2n+1 OH. We can also write this as Cn H2n+2 O.
2. Substitute the atomic masses: (12 * n) + (1 * (2n + 2)) + 16 = 60.
3. 12n + 2n + 2 + 16 = 60; 14n + 18 = 60.
4. 14n = 60 - 18; 14n = 42. Therefore, n = 42 / 14 = 3.
5. The formula is C3 H7 OH (Propanol).
Key points
- •Functional group is the hydroxyl (-OH) group.
- •General formula is Cn H2n+1 OH.
- •Classified as primary, secondary, or tertiary.
- •Boiling points are high due to hydrogen bonding.
- •Primary alkanols oxidize to form alkanoic acids.