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# 2.2: Functional group

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##### Learning Objectives
• Recognize the two basic mechanisms of bond breaking and making and draw curly arrows to show the electron movement in these steps.
• Define organic functional group and recognize it in an organic compound.

## What is a chemical reaction?

A chemical reaction involves making and breaking chemical bonds. Often the elemental composition of the compound changes during a chemical reaction. Sometimes the composition remains the same; atoms rearrange, resulting in a new combination that is an isomer of the initial compound. Often making and breaking of a covalent bond is involved in organic reactions. Recall that a covalent bond is a shared pair of electrons. The bond breaking or making can happen in one of two modes: homolytic or heterolytic, as described below.

### Homolytic breaking and bond-making

In homolytic bond breaking, each bonded atom receives one of the two electrons in a covalent bond, e.g.:

, where half-headed arrows show the movement of a single electron. The above reaction produces two atoms with one unpaired electron, i.e., free radicals. The reverse of it is hemolytic bond making, e.g.:

### Heterolytic bond-making and breaking

In heterolytic bond breaking, the bonding electrons move to one of the bonded atoms, usually to a more electronegative atom, making a lone pair, e.g.:

, where the regular double-headed arrows show the movement of an electron pair. The formal charge changes in heterolytic bond-making and breaking. The bonded atom that receives the electrons usually becomes an anion, and the other becomes a cation in the above reaction. Revers of it is heterolytic bond making, e.g.:

### What is a functional group?

A functional group is an atom or a group of atoms that imparts a characteristic set of physical and chemical properties to the compound.

For example, alkanes with no functional group are the least reactive classes of organic compounds. Alkanes have only $$\ce{C-C}$$ and $$\ce{C-H}$$ single bonds, which are nonpolar and strong bonds. Alkenes, alkynes, and aromatic compounds are relatively reactive classes of organic compounds because they have a $$\ce{C=C}$$ bond, $$\ce{C≡C}$$ bond, or a benzene ring as functional groups. Although alkenes, alkynes, and aromatic hydrocarbons are nonpolar, there is a partial negative ($$\delta^{-}$$) character in the $$\pi$$-bond region that attracts electrophilic reagents having $$\delta^{+}$$ regions. Further, the $$\pi$$-bond is weaker than a $$\sigma$$-bond making it easier to break during chemical reactions.

## Classification of organic compounds based on the functional groups

One primary class of organic compounds is hydrocarbons that contain only $$\ce{C's}$$ and $$\ce{H's}$$, listed in Table 1, and the others are organic compounds with heteroatom/s, in their functional group/s. Hydrocarbons have been introduced in the previous section. Other classes of organic compounds containing heteratom\s in their functional groups are described next.

The heteroatom/s in the functional group can be a single bonded, i.e., sp3-hybridized halogen, $$\ce{O}$$, $$\ce{N}$$, $$\ce{S}$$, $$\ce{P}$$, etc., listed in Table 2; a double bonded, i.e., sp2-hybridized $$\ce{O}$$, $$\ce{N}$$, $$\ce{S}$$, $$\ce{P}$$, etc., listed in Table 3; or a combination of these, listed in Table 4.

Table 1: Classes of hydrocarbons based on their functional groups. (R-, R1-, R2, R3, and R4- represents a general alkyl group or hydrogen)
Class name Group name General structural formula General condensed formula Prefix Suffix Example
Alkane Alkyl R R alkyl- -ane
Alkene Alkenyl $$\ce{R2C=CR2}$$ alkenyl- -ene
Alkyne Alkynyl $$\ce{RC≡CR2}$$ alkynyl- -yne
Aromatic (benzene derivatives) Phenyl $$\ce{R-C6H5}$$ phenyl- -benzene
Table 2: Classes of organic compounds containing an sp3-hybridized heteroatom in functional groups. (R-, R1-, R2, R3, and R4- represent a general alkyl group or hydrogen.)
Class name Group name General structural formula General condensed formula Prefix Suffix Example
Haloalkane Halo $$\ce{R-X}$$, i.e., $$\ce{R-F}$$, $$\ce{R-Cl}$$, $$\ce{R-Br}$$, or $$\ce{R-I}$$ $$\ce{R-X}$$ i.e., $$\ce{R-F}$$, $$\ce{R-Cl}$$, $$\ce{R-Br}$$, or $$\ce{R-I}$$ halo- i.e., fluoro-, chloro-, bromo-, or iodo-) - $$\ce{CH3-CH2-Cl}$$ (chloroethane)
Alcohol Alcohol $$\ce{R-OH}$$ $$\ce{ROH}$$ hydroxy- -ol $$\ce{CH3-CH2-OH}$$ (ethanol)
Phenol Phenol $$\ce{C6H5OH}$$ - -phenol
Ether Ether $$\ce{R-O-R'}$$ $$\ce{ROR'}$$ Alkoxy - $$\ce{CH3-CH2-O-CH2-CH3}$$ (ethoxyethane)
Epoxide Epoxide epoxy- -oxirane
Peroxide Peroxy $$\ce{R-O-O-R'}$$ $$\ce{ROOR'}$$ peroxy- alkyl peroxide
Thiol Sulfhydryl $$\ce{R-S-H}$$ $$\ce{RSH}$$ sulfanyl- -thiol
Sulfide (Thioether) Sulfide $$\ce{R-S-R'}$$ $$\ce{RSR'}$$ sulfanyl- -sulfide
Disulfide Disulfide $$\ce{R-S-S-R'}$$ $$\ce{RSSR'}$$ disulfanyl- -disulfide
Amine Amine $$\ce{R1R2R3N}$$ amino- -amine
Aniline Aniline $$\ce{C6H5NH2}$$ - -aniline
Table 3: Classes of organic compounds containing an sp2-hybridized heteroatom in functional groups. (R-, R1-, R2, R3, and R4- represents a general alkyl group or hydrogen)
Class name Group name General structural formula General condensed formula Prefix Suffix Example
Aldehyde Aldehyde $$\ce{RCHO}$$ oxo- -al
Benzaldehyde Benzaldehyde $$\ce{C6H5COH}$$ - -benzaldehyde
Ketone Ketone $$\ce{RCOR'}$$ oxo- -one
Imine Imine $$\ce{R1C(=NR3)R2}$$ imino- -imine

Table 4: Classes of organic compounds containing a mix of sp3-, sp2-, and sp-hybridized heteroatoms in functional groups. (R-, R1-, R2, R3, and R4- represents a general alkyl group or hydrogen)

Class name Group name General structural formula General condensed formula Prefix Suffix Example
Carboxylic acid Carboxyl $$\ce{R-COOH}$$ carboxy- oic acid
Acyl halide Acyl halide $$\ce{R-COX}$$ - oil halide
Acid anhydride Acid anhydride $$\ce{R1-(CO)O(CO)R2}$$ - R1-oil R2-oat, or R1-oic R2-oic anhydride
Easter Easter $$\ce{R1COORR'}$$ - alkyl -alkanoate
Amide Amide $$\ce{RCONR'R"}$$ alkyl carbamoyl -amide
Carboxylate Carboxylate $$\ce{RCOO^{-}}$$ -carboxy -oate
Nitrile Nitrile $$\ce{R-C≡N}$$ $$\ce{RCN}$$ cyano- -nitrile
Phosphate Phosphate $$\ce{ROP(=O)(OH)2}$$ - phosphate
Nitro Nitro $$\ce{RNO2}$$ nitro- -

2.2: Functional group is shared under a Public Domain license and was authored, remixed, and/or curated by LibreTexts.

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