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In [[organic chemistry]], '''butyl''' is a four-[[carbon]] [[alkyl]] [[radical (chemistry)|substituent]] with [[chemical formula]] '''-C<sub>4</sub>H<sub>9</sub>''' .  It is derived from either of the two isomers of the [[alkane]] called [[butane]].

Each of the two [[isomer]]s of butane give rise to two [[isomers]] of the butyl substituent.  Thus, ''n''-butane can connect at either the terminal or an internal carbon atoms, giving rise to "''n''-butyl" and "''sec''-butyl" substituents.

* '''''n''-butyl''' group; <big>CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>&ndash;</big>, (fully systematic name "butyl")
* '''''sec''-butyl''' group; <big>CH<sub>3</sub>CH<sub>2</sub>(CH<sub>3</sub>)CH&ndash;</big>, (fully systematic name "1-methylpropyl")

The second, branched isomer of butane, [[Isobutyl|isobutane]] can also connect either terminal methyl or internal carbon atoms, giving rise to "isobutyl" and "tertiary butyl" substituents, respectively.

* '''isobutyl''' group; <big>(CH<sub>3</sub>)<sub>2</sub>CHCH<sub>2</sub>&ndash;</big>, (fully systematic name "2-methylpropyl")
* '''''tert''-butyl''' group; <big>(CH<sub>3</sub>)<sub>3</sub>C&ndash;</big>, (fully systematic name "1,1-dimethylethyl")

==Nomenclature==

According to [[IUPAC nomenclature]], "isobutyl", "''sec''-butyl", and "''tert''-butyl" are all retained trivial names.

{| class="wikitable"
|[[skeletal formula]]
|[[common name]]
|IUPAC name
|[[systematic name]]
|alternate notation
|-
|[[Image:Butyl-group-2D-skeletal.png|100px]]
|''n''-butyl
|butyl
|butyl
|butan-1-yl
|-
|[[Image:Isobutyl-group-2D-skeletal.png|80px]]
|isobutyl
|isobutyl
|2-methylpropyl
|2-methylpropan-1-yl
|-
|[[Image:Sec-butyl-group-2D-skeletal.png|80px]]
|''sec''-butyl
|''sec''-butyl
|1-methylpropyl
|butan-2-yl
|-
|[[Image:Tert-butyl-group-2D-skeletal.png|60px]]
|''tert''-butyl
|''tert''-butyl
|1,1-dimethylethyl
|2-methylpropan-2-yl
|}

Butyl is the largest substituent for which trivial names are commonly used for all isomers.

The prefixes iso, <i>sec</i> and <i>tert</i> refer to the number of carbons connected to the [[primary carbon]] (also known as R<sup>I</sup> ("R prime"), the carbon that is connected to R). Iso means one, <i>sec</i>- means two and <i>tert-</i> means three.

==Some examples==

The following are the four isomers of "butyl acetate":

{|
|[[Image:Butyl acetate.png|200px|butyl acetate]] 
|[[Image:Isobutyl acetate.png|150px|isobutyl acetate]]
|[[Image:S-butyl acetate.png|150px|sec-butyl acetate]]
|[[Image:T-butyl acetate.png|120px|tert-butyl acetate]]
|-
|<center>[[Butyl acetate|''n''-butyl acetate]]</center>
|<center>[[isobutyl acetate]]</center>
|<center>[[Sec-Butyl acetate|''sec''-butyl acetate]]</center>
|<center>[[Tert-Butyl acetate|''tert''-butyl acetate]]</center>
|}

== Etymology==
As the number of carbons in an alkyl chain increases, butyl is the last to be named historically instead of through [[Greek language|Greek]] numbers. The name is derived from [[butyric acid]], a four carbon [[carboxylic acid]] found in [[decomposition|rancid]] [[butter]]. The name of butyric acid, in turn, comes from [[Latin]] ''butyrum'', "butter".

==Tert-butyl effect==
The tert-butyl [[substituent]] is very bulky and used in chemistry for [[kinetic reaction control|kinetic stabilisation]] together with other bulky groups such as the related [[trimethylsilyl]] group. The effect that the t-butyl group exerts on the progress of a chemical reaction is called the '''tert-butyl effect'''. 

This effect is illustrated in the [[Diels-Alder reaction]] below where the tert-butyl substituent causes a [[reaction rate]] acceleration by a factor of 240 compared to hydrogen as the substituent.<ref>''Factors affecting ease of ring formation. The effect of anchoring substitution on the rate of an intramolecular diels-alder reaction with furan-diene'' Serge Cauwberghs and Pierre J. De Clercq B. Tinant and J. P. Declercq [[Tetrahedron Letters]] Volume 29, Issue 20 , '''1988''', Pages 2493-2496 {{doi|10.1016/S0040-4039(00)87916-2}}</ref>

[[Image:Tertbutyleffect.png|center|400px|Tert-butyl effect]]

== See also ==
* [[methyl]]
* [[ethyl]]
* [[propyl]]
* [[pentyl]] / [[amyl]]

==References==
<div class="references-small"><references/></div>

[[Category:Functional groups]]

[[pl:Grupa butylowa]]
[[ru:Бутил]]

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