Substituent facts for kids

A substituent is a single atom or a group of atoms that takes the place of another atom in a molecule. Think of it like swapping out one building block for another in a chemical structure. This kind of chemical swap is called a substitution reaction. The atom or group of atoms that gets replaced is known as a leaving group.

Words like side chain, group, or branch are often used in a similar way to describe parts that stick out from a main chemical structure. For example, in very large molecules called polymers (like plastics), side chains extend from the main "backbone" of the molecule. In proteins, these parts are attached to specific carbon atoms in the amino acid chain.

In organic chemistry, there are special rules for naming compounds that have substituent groups. The name of the substituent usually comes first, with a special ending (called a suffix) that tells us how it's connected to the main carbon chain.

• The suffix -yl is used when a single hydrogen atom is replaced by a single bond.
• -ylidene is used for double bonds.
• -ylidyne is used for triple bonds.

When naming hydrocarbons (molecules made of hydrogen and carbon) with substituents, numbers are often used. These numbers show exactly which carbon atom the substituent is attached to. This is especially important for isomers, which are molecules with the same chemical formula but different arrangements of atoms.

Scientists also use terms like most-substituted and least-substituted to describe molecules. This helps them predict how molecules will react. For example:

• Markovnikov's rule helps predict where a hydrogen atom will add to a double bond. It usually adds to the carbon that already has more hydrogen atoms attached.
• Zaitsev's rule predicts that the main product of a reaction will be the alkene (a type of hydrocarbon with a double bond) that has more substituents attached to its double bond, because this makes it more stable.

How Substituents are Shown

In a diagram of a molecule, the letters R or X are often used as placeholders for substituent groups.

• The letter R (or R¹, R², etc.) is a general symbol for an organic substituent. This could be a methyl group, an ethyl group, or an aryl group. The "R" comes from words like "radical" or "rest." It can represent any number of atoms. Charles Frédéric Gerhardt first used this symbol in 1844.
• The symbol X is often used to represent electronegative substituents. These are atoms that tend to attract electrons, like the halides (such as fluorine, chlorine, bromine, or iodine).

Related Topics

• Side chain
• Functional groups are a type of substituent that give molecules specific chemical properties.

See also

In Spanish: Sustituyente para niños