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Substituent
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<p>In <a href="/facts/Organic_chemistry/djxhxaNg">organic chemistry</a>, a substituent is one or a group of <a href="/facts/Atom/FoQ4kGN5">atoms</a> that replaces (one or more) atoms, thereby becoming a <a href="/facts/Moiety_(chemistry)/WxKqcJe1">moiety</a> in the resultant (new) <a href="/facts/Molecule/N9ljSfFq">molecule</a>. </p><p>The suffix <i>-yl</i> is used when naming <a href="/facts/Organic_compound/gqCNaN45">organic compounds</a> that contain a <a href="/facts/Single_bond/8qz02fEl">single bond</a> replacing one hydrogen; <i>-ylidene</i> and <i>-ylidyne</i> are used with <a href="/facts/Double_bond/qDemMn0c">double bonds</a> and <a href="/facts/Triple_bond/poDWRayX">triple bonds</a>, respectively. In addition, when naming hydrocarbons that contain a substituent, positional numbers are used to indicate which carbon atom the substituent attaches to when such information is needed to distinguish between <a href="/facts/Isomer/PXlvDVdg">isomers</a>. Substituents can be a combination of the <a href="/facts/Inductive_effect/CN4c8VRt">inductive effect</a> and the <a href="/facts/Mesomeric_effect/W8aLIlBw">mesomeric effect</a>. Such effects are also described as <a href="/facts/Electron-rich/GDzV1DXL">electron-rich</a> and <a href="/facts/Electron_withdrawing/GDzV1DXL">electron withdrawing</a>. Additional <a href="/facts/Steric_effects/TnX5c8HG">steric effects</a> result from the volume occupied by a substituent. </p><p>The phrases <i>most-substituted</i> and <i>least-substituted</i> are frequently used to describe or compare molecules that are <a href="/facts/Product_(chemistry)/8uCoDUdv">products</a> of a <a href="/facts/Chemical_reaction/GpqWlKrz">chemical reaction</a>. In this terminology, <a href="/facts/Methane/kBHneYnh">methane</a> is used as a reference of comparison. Using methane as a reference, for each hydrogen atom that is replaced or "substituted" by something else, the molecule can be said to be more highly substituted. For example: </p> <ul><li><a href="/facts/Markovnikov%2527s_rule/WJokRx6l">Markovnikov's rule</a> predicts that the hydrogen atom is added to the carbon of the <a href="/facts/Alkene/RtYcS6kU">alkene</a> functional group which has the greater number of hydrogen atoms (fewer <a href="/facts/Alkyl/vmEk8X4v">alkyl</a> substituents).</li> <li><a href="/facts/Zaitsev%2527s_rule/s4bmSAbE">Zaitsev's rule</a> predicts that the major reaction product is the alkene with the more highly substituted (more stable) double bond.</li></ul>