Functional Groups - Naming, Properties and Reactivities

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A  functional group is a  group of atoms within a compound that is responsible for the characteristic chemical reactions of that compound. Functional groups also play an important part in organic compound nomenclature -  combining the names of the functional groups with the names of the parent alkanes provides a way to distinguish compounds.

Some Common Functional Groups:

I.  Alcohols

Alcohols are functional groups characterized by the presence of an -OH group. The general formula for alcohol is :  R – OH  where R is an alkyl . Compared to their parent alkanes,  alcohols  have  higher boiling points. They are polar in nature caused by the difference in electronegativities between carbon and the oxygen atoms. In a reaction, alcohols cannot leave the molecule by themselves. To leave a molecule, they must be protonated to water. In the presence of a strong base, alcohols can be deprotonated.

Nomenclature of Alcohols

    In naming alcohol, the terminal “-e” of the parent carbon chain (alkane, alkene, or alkyne) is dropped and the addition of “-ol” as the ending.

Example:          CH3CH2-    +       OH           CH3CH2-OH
                    ethyl          +       OH            ethanol ( or ethyl alcohol )
Alcohols are used  in beverages, antifreeze, antiseptics, and fuels. They are also used as preservatives for specimens in science, and they are used as reagents and solvents in industries because they can  dissolve both polar and non-polar substances.

II.  Ethers

Ethers are organic compounds that contain an ether group. An ether group is an oxygen atom connected to two alkyl or aryl groups. They follow the general formula      R-O-R’ ( R is a substituent that can be the same or not ). The C-O-C linkage is characterized by bond angles of 104.5 degrees, with the C-O distances being about 140 pm. The oxygen of the ether is more electronegative than the carbons. Thus, the alpha hydrogens are more acidic than in regular hydrocarbon chains.
Ethers are nonpolar due to the presence of an alkyl group on either side of the central oxygen. The oxygen atom is unable to participate in hydrogen bonding due to the presence of alkyl groups on its side. Ethers have lower boiling points . However, as the alkyl chain of the ethers becomes longer, the difference in boiling points becomes smaller. This is due to the effect of increased Van der Waals interactions as the number of carbons increases, and therefore the number of electrons increases as well. The two lone pairs of electrons present on the oxygen atoms make it possible for ethers to form hydrogen bonds with water. Ethers are more polar than alkenes, but not as polar as esters, alcohols or amides of comparable structures.
Ethers have relatively low chemical reactivity. They resist undergoing hydrolysis. Ethers form peroxides in the presence of oxygen or air.


Nomenclature of Ethers

In naming , identify the alkyl groups on either side of the oxygen atom in alphabetical order, then write “ether.”
Examples:   a)        CH3 – O – CH2CH3        ethyl methyl ether
                    b)        CH3 – O – CH3        dimethyl ether

Note: If the two alkyl groups are identical, the ether is called di[alkyl] ether.

III.  Ketones

Ketone is an organic  compound containing an oxygen atom joined to a carbon atom by a double bond. In ketone, the carbonyl functional group ( C=O ) is placed within a molecule. Its general structure is  :           O             where R and  R’ can be
                         R – C – R’
a variety of carbon-containing substituents.
Ketones are polar due to the carbonyl group and can interact with other compounds through hydrogen bonding. This hydrogen bonding makes ketones more soluble in water . Ketones are not hydrogen bond donors and they exhibit intermolecular attractions with other ketones. Because of this, ketones are often more volatile than alcohols and carboxylic acids of comparable molecular weights.

Nomenclature of  Ketones

Naming of ketone is done  by changing the suffix of the parent carbon molecule to “-one.” If the position of the ketone must be specified, parent chain is numbered by giving the lowest number to  the carbonyl ( C=O ).
   
Example:                   CH2CH3
           
                      CH3 – C – C – CH3         3-ethyl-3-methyl -2- butanone
                       4         3    2      1
                                        O
                                             CH3

IV.    Aldehydes

An aldehyde is an organic compound that contains a carbonyl group ( C=O ) with the central carbon bonded to a hydrogen and R group (R-CHO). In  aldehydes,  the carbonyl is placed at the end of the carbon skeleton rather than between two carbon atoms of the backbone ( just like in  ketone ). Like ketones, aldehydes are sp2 hybridized.

Nomenclature of  Aldehydes

 Aldehydes are named by dropping the suffix “e” of the parent alkane, and adding the suffix “-al”.
    Example:     aldehyde with 2 C atoms
                                   H
 
                           CH3C =O        ethanal


Ketones and aldehydes can be readily reduced to alcohols  in the presence of a strong reducing agent such as sodium borohydride. In the presence of strong oxidizing agents, they can be oxidized to carboxylic acids.

V.     Carboxylic Acids

Carboxylic acids are organic acids that contain a carbon atom that participates in both a hydroxyl and a carbonyl functional group. Its general formula is : R-COOH.
A carboxyl group (COOH) is a functional group consisting of a carbonyl group (C=O) with a hydroxyl group (O-H) attached to the same carbon atom. Carboxyl groups have the formula        O           . It is usually written  as  – COOH.  Carboxylic  acids  are
                    -  C – OH
characterized by the presence of one carboxyl group. Since carboxylic acids contain both hydroxyl and carbonyl functional groups, they participate in hydrogen bonding as both hydrogen acceptors and hydrogen donors. Carboxylic acids increase stabilization of non-polar compounds and elevate their boiling points.

Nomenclature of  Carboxylic Acids
For IUPAC nomenclature, the suffix “e” of the parent alkane is dropped and the suffix “-oic acid”  is added.
                                               O
   
Example:    CH3 –  C – OH            ethanoic acid

Carboxylic acids are used as precursors to form other compounds such as esters, aldehydes, and ketones.
Carboxylic acids are used in the production of polymers, pharmaceuticals, solvents, and food additives.  Carboxylic acids are generally produced from oxidation of aldehydes and hydrocarbons, and base catalyzed dehydrogenation of alcohols.

VI.    Esters

Esters are functional groups produced from the condensation of an alcohol with a carboxylic acid. The general formula is:       O  
                                                   R – C – OR’

R and R’ are both alkyl groups. Esters are derivative of carboxylic acids where the hydroxyl (OH) group has been replaced by an alkoxy (O-R) group. They are commonly synthesized from the condensation of a carboxylic acid with an alcohol.
Esters are more polar than ethers, but less  than alcohols. They participate in hydrogen bonds as hydrogen bond acceptors, but cannot act as hydrogen bond donors.  Esters are more volatile than carboxylic acids of similar molecular weight.
Esters are everywhere. Most naturally occurring fats and oils are the fatty acid esters of glycerol. Esters are fragrant, and those with low molecular weights are commonly used as perfumes and are found in essential oils and pheromones. Polymerized esters, or polyesters, are important plastics.

Nomenclature of  Esters

Ester names are derived from the parent alcohol and acid. For example, the ester formed by ethanol and ethanoic acid is known as ethyl ethanoate; “ethanol” is reduced to “ethyl,” while “ethanoic acid” is reduced to “ethanoate.”
                     O                            O

    Illustration:    CH3 – C – OH  +  CH3CH2 – OH              CH3-CO-CH2CH3 
                                ethanoic acid           ethanol                        ethyl ethanoate   


VII.    Amines

Amines are compounds characterized by the presence of a nitrogen atom, a lone pair of electrons, and three substituents ( R ).  The general formula for amines is N-R3. R can be H ( hydrogen ) or alkyl groups. Amines are basic compounds due to the lone pair of electrons. Amines have an unpleasant odor
Amine compounds can hydrogen bond, which affords them solubility in water and elevated boiling points. Amines are reactive due to their basicity as well as their nucleophilicity. Most primary amines are good ligands and react with metal ions to yield coordination complexes
Amines are starting materials for dyes and models for drug design. Amines are also used for gas treatment in the removal  CO2 from combustion gases.

Nomenclature of  Amines

    In naming an amine compound, the prefix “amino-” or the suffix “-amine” is used.
For organic compound with multiple amino groups, the prefixes di, tri, tetr , etc. are used.
                                              . .
    Examples: a)    CH3 – N – CH3        trimethylamine
                     CH3

                      . .
              b)  CH3CH2 – N – H    ethanamine or ethyl amine
                              H

VIII.    Amides

    Amides  are organic compounds containing  nitrogen and has the general formula ;       O           
                  RC – NH2
    The – NH2 group is directly attached to the carbonyl group ( C=O ).
    Amide groups are found in nylon, silk, and wool. Amide is also present in insect repellant.  Amides are used widely as color  in crayons, pencils and inks, paper industry , plastic and rubber industry, and water and sewage treatment.
Nomenclature of  Amide
    In naming  an amide compound,  the suffix “e” of the parent alkane is replaced with the suffix “amide.
                                       O
    Example:    CH3CH2CH2CH2C – NH2        pentanamide

IX.    Alkyl halides

Alkyl halides are organic compounds containing halogens ( fluorine, chlorine, bromine or iodine ). Alkyl halide is formed by replacing one or more hydrogen atoms in an alkane with halogen atoms.They are also known as haloalkanes. Alkyl halides have the following general formula:
    Primary alkyl halide    :    R – CH2 – X
    Secondary alkyl halide:    R – CH – R
                                    X
                                                          R
Tertiary alkyl halide     :    R – C –R
                           X
               
     where X is any halogen mentioned above.

Halogen imparts reactivity to alkyl halides. Alkanes impart odorlessness and colorlessness to alkyl halides. Some alkyl halides are less toxic and have high heat of vaporization. They are water-phobic ( they repel water) . Halogenated hydrocarbons are soluble in organic solvents. Some of the haloalkanes do not conduct electricity. Alkyl halides have higher boiling and melting point unlike alkanes. Haloalkanes are less flammable as compared to its component alkanes. Alkyl halides have stronger intermolecular forces (dipole-dipole interaction.)

Nomenclature of  alkyl halides

In naming alkyl halides, the name of the alkyl residue is followed by the name of the halide. Examples are methyl iodide and ethyl chloride.
The IUPAC nomenclature considers an alkyl halide a substituted alkane . The name of the halogen is followed by the name of the alkane. Examples are iodomethane and chloromethane. . If an alkyl halide contains more than one halogen, the halogen names are noted in alphabetical order, such as 1-chloro-2-iodobutane.

Illustrations:
    a)    CH3 – Cl    methyl chloride or chloromethane
    b)    CH3-CH2-F      ethyl fluoride or fluoroehtane
    c)    CH3-CH-CH3   isopropyl iodide or 2-iodopropane
                 I   
    d)    CH3-CH2-CH-CH3   sec-butyl bromide or 2-bromobutane
                  Br
    e)                 CH3
                  H3C -  C – CH3   tert-butyl bromide or 2-bromo-2-methylpropane
                      Br

Alkyl halides are used in labs as synthetic intermediate compounds.  They are used as cleansers for cleaning. Commercial uses of haloalkanes include its use in fire extinguishers. Carbon tetrachloride is used to detect neutrinos.