Alcohols (Organic chemistry)
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What is an alcohol?
Alcohols are the family of compounds that contain one or more hydroxyl (-OH) groups. Alcohols are represented by the general formula R-OH. Alcohols are important in organic chemistry because they can be converted to and from many other types of compounds. Reactions with alcohols fall into two different categories. Reactions can cleave the R-O bond or they can cleave the O-H bond.
The alcohol in alcohol beverages is ethanol, CH3CH2OH.
Naming alcohols
Follow these rules to name alcohols the IUPAC way:
- find the longest carbon chain containing at least one OH group, this is the parent
- if there are multiple OH groups, look for the chain with the most of them, and the way to count as many carbons in that chain
- name as an alcohol, alkane diol, triol, etc.
- number the OH groups, giving each group the lowest number possible when different numbering possibilities exist
- treat all other groups as lower priority substituents (alcohol / hydroxy groups are the highest priority group for naming)
Example alcohols
IUPAC name |
Common name
| |
| CH3CH2OH | Ethanol | Ethyl alchohol |
| CH3CH2CH2-OH | 1-Propanol | n-propylalcohol |
| CH3CH-OH-CH3 | 2-Propanol | Isopropyl alcohol (Note: Isopropanol would be incorrect. Cannot mix and match between systems.) |
| 2-Ethylbutan-1-ol | 2-Ethyl-butanol | |
| 3-Methyl-3-pentanol | ||
| 2-2-Dimethylcyclopropanol | ||
| Multiple OH functional groups | ||
| 1,2-Ethanediol | Ethylene glycol | |
| 1,1-Ethanediol | Acetaldehyde hydrate | |
| 1,4-Cyclohexanediol | ||
| (from that body fat is stored as) | 1,2,3-Propanetriol | Glycerol |
| -OH can be named as a substituent hydroxyl group (hydroxyalkanes) | ||
| 1,2-Di(hydroxymethyl)cyclohexane | ||
| 2-(hydroxymethyl)-1,3-propanediol | ||
| Find the longest chain of carbons containing the maximum number of -OH groups | ||
| 1-(1-hydrozyethyl)-1-methylcyclopropane | ||
| 2-(1-methylcyclopropyl)ethanol | ||
| 2-(2-hydroxyethyl)-2-methylcyclopropanol | ||
| 3-(2-hydroxyethyl)-3-methyl-1,2-cyclopropanediol |
Properties of alcohols
Acidity of alcohol
Review general topics of acidity and basicity here.
In an O-H bond, the O steals the H's electron due to the polar nature of the O-H bond, and O doesn't mind carrying a negative charge. This leads to deprotonation in which the nucleus of the H, a proton, leaves completely. This makes the -OH group (and alcohols) Bronsted acids. Alcohols are weak acids, even weaker than water. Ethanol has a pKa of 15.9 compared to water's pKa of 15.7. The larger the alcohol molecule, the weaker an acid it is.
Alkoxides
When O becomes deprotonated, the result is an alkoxide. Alkoxides are anions. The names of alkoxides are based on the original molecule. (Ethanol=ethoxide, butanol=butoxide, etc.)
Producing an alkoxideR-OH -> H+ + R-O- In this equation, R-O- is the alkoxide produced and is the conjugate base of R-OH |
Alcohols can be converted into alkoxides by reaction with a strong base (must be stronger than OH-) or reaction with metallic sodium or potassium. Alkoxides themselves are basic. The larger an alkoxide molecule is, the more basic it is.
Conversion of alcohols to haloalkanes
Recall that haloalkanes can be converted to alcohols through nucleophilic substitution.
Conversion of a haloalkane to an alcoholR-X + OH- → R-OH + Br- |
This reaction proceeds because X (a halogen) is a good leaving group and OH- is a good nucleophile. OH, however, is a poor leaving group. To make the reverse reaction proceed, OH must become a good leaving group. This is done by protonating the OH, turning it into H2O+, which is a good leaving group. H+ must be present to do this. Therefore, the compounds that can react with alcohols to form haloalkanes are HBr, HCl, and HI. Just like the reverse reaction, this process can occur through SN2 (backside attack) or SN1 (carbocation intermediate) mechanisms.
SN2 conversion of an alcohol to a haloalkaneR-O-H + H+ + X- → R-O+-H2 + X- → R-X + H2O |
SN1 conversion of an alcohol to a haloalkaneR-O-H + H+ + X- → R-O+-H2 + X- → R+- + H2O + X- → R-X + H2O |
Remember, the two mechanisms look similar but the mechanism affects the rate of reaction and the stereochemistry of the product.
Oxidation of alcohols
Oxidation in organic chemistry always involves either the addition of oxygen atoms or the removal of hydrogen atoms. Whenever a molecule is oxidized, another molecule must be reduced. Therefore, these reactions require a compound that can be reduced. These compounds are usually inorganic. They are referred to as oxidizing reagants.
With regards to alcohol, oxidizing reagants can be strong or weak. Weak reagants are able to oxidize a primary alcohol group into a aldehyde group and a secondary alcohol into a ketone. Stong reagants will further oxidize the aldehyde into a carboxylic acid. Tertiary alcohols cannot be oxidized.
An example of a strong oxidizing reagant is chromic acid (H2CrO4). An example of a weak oxidizing reagant is Pyridinium chlorochromate (C5H6NCrO3Cl, aka PCC)
General reactions:
Ethers
Ethers are a derivative of alcohols. The functional group of ethers is R-O-R. Ethers can be viewed as a water molecule in which both H atoms are replaced with alkyl groups. Ethers may exist in straight chain carbons (acyclic) or as part of a carbon ring (cyclic). They also have a distinct pleasant smell.
Acyclic ethers
Naming acyclic ethers
Think of one side of the O of the ether as its own substituent. The name of the ether is based off the name of the alkoxide that substituent resembles by replacing -ide with -y. For example, CH3O would be called methoxy.
Synthesis of acyclic ethers
Most acyclic ethers can be prepared using Williamson synthesis. This involves reacting an alkoxide with a haloalkane. Remember, alkoxides are created by reacting an alcohol with metallic sodium or potassium.
Cleavage of acyclic ethers
Cyclic ethers
Naming cyclic ethers
Synthesis of cyclic ethers
Cleavage of cyclic ethers
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