alcohol

What is an Alcohol?

Sitting in a bar drinking a beer, have you ever wondered, ''What is in this drink that makes me feel so relaxed?'' You probably already know the answer is alcohol, specifically ethanol, but have you ever wondered what exactly alcohol is? Alcohol molecules are organic molecules that contain an -OH group. This -OH group makes the molecule reactive, so it is called a functional group.

Alcohol functional groups are found in biological molecules such as sugars, amino acids, and vitamins. Alcohol functional groups are also found in molecules that are used every day. You can find ethanol in drinks like wine, beer, vodka, whiskey, gin, and rum, among many others. Ever had to check the antifreeze levels in your car? It contains ethylene glycol, another alcohol.

Structure of ethylene glycol

Nomenclature

The alcohol functional group hydroxyl is an -OH group. When naming alcohols, the -e is removed from the hydrocarbon name and an -ol suffix is added. A number in front of the hydrocarbon name tells the location of the alcohol group.

For example, in 1-butanol, there are four carbon atoms, so the parent hydrocarbon name is butane. The -e is dropped from butane and -ol is added, resulting in butanol. The 1 indicates the alcohol group is on the first carbon.

1-butanol

The carbon chain should be numbered so that the alcohol has the lowest number. When naming the molecule 1-butanol, you can either name it as 4-butanol or 1-butanol, depending on which side of the molecule you begin to count. Since 1 is a smaller number than 4, 1-butanol is correct. If there are two alcohol groups on a molecule the suffix is changed to -diol.

Alcohol Molecule Classification

Molecules can be classified based on the number of alcohol groups.

• monohydric -one alcohol group on the molecule
• dihydric - two alcohol groups on the molecule
• polyhydric - more than 2 alcohol groups on the molecule

Alcohol classifications are based on the number of alcohol groups

Solubility

The number of alcohol groups that a molecule has will affect its solubility. As the number of alcohol groups increase on the molecule with the same number of carbons, the solubility in water increases. For example, 1,4-butandiol is more soluble in water than 1-butanol.

1-butanol (above) is less soluble than 1,4-butandiol, because it has fewer alcohol groups

Another way to think about this is that as the number of carbon atoms increase in an alcohol, the solubility in water decreases. So, 1-octanol with eight carbons per alcohol group is less soluble in water than 1-butanol with four carbons per alcohol group.

Alcohol Functional Group Classification

Alcohols can also be classified based on the carbon to which they are attached.

• primary - the alcohol is attached to a carbon with two hydrogens
• secondary - the alcohol is attached to a carbon with one hydrogen
• tertiary - the alcohol is attached to a carbon atom with no hydrogens

Examples of primary, secondary, and tertiary alcohols

UPDATE 

 Alcohols are one of the most important functional groups in organic chemistry. Alcohols are a good source of reagents for synthesis reactions. The ability to identify alcohols is important especially when looking at IR and NMR spectra. The alcohol signal is very easy to spot on IR graphs, because they have a strong signal near the 3200 cm-1 region.

Introduction

The following is list of some common primary alcohols based on the IUPAC naming system.

Name	Molecular Formula
Methanol (methyl alcohol)	CH3OH
Ethanol (ethyl alcohol)	C2H­­­­5OH
Propanol	C3H7OH
Butanol	C4H9OH
Pentanol	C5H11OH
Hexanol	C6H13OH
Heptanol	C7H15OH
Octanol	C8H17OH

Rules for naming the alcohols

1. Find the longest chain containing the hydroxy group (OH). If there is a chain with more carbons than the one containing the OH group it will be named as a subsitutent.
2. Place the OH on the lowest possible number for the chain. With the exception of carbonyl groups such as ketones and aldehydes, the alcohol or hydroxy groups have first priority for naming.
3. When naming a cyclic structure, the -OH is assumed to be on the first carbon unless the carbonyl group is present, in which case the later will get priority at the first carbon.
4. When multiple -OH groups are on the cyclic structure, number the carbons on which the -OH groups reside.
5. Remove the final e from the parent alkane chain and add -ol. When multiple alcohols are present use di, tri, et.c before the ol, after the parent name. ex. 2,3-hexandiol. If a carbonyl group is present, the -OH group is named with the prefix "hydroxy," with the carbonyl group attached to the parent chain name so that it ends with -al or -one.

Examples

Ethane: CH3CH3 ----->Ethanol:   (the alcohol found in beer, wine and other consumed sprits)

Secondary alcohol:  2-propanol

Other functional groups on an alcohol:  3-bromo-2-pentanol

Cyclic alcohol (two -OH groups):  cyclohexan-1,4-diol

Other functional group on the cyclic structure:  3-hexeneol (the alkene is in bold and indicated by numbering the carbon closest to the alcohol)

A complex alcohol:  4-ethyl-3hexanol (the parent chain is in red and the substituent is in blue)

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 GOODNIGHT