Butyl group#Tert-butyl as Protecting group
{{Short description|Chemical group (−C₄H₉) derived from butane}}
{{Redirect|Butyl|the synthetic rubber|Butyl rubber}}
In organic chemistry, butyl is a four-carbon alkyl radical or substituent group with general chemical formula {{chem2|\sC4H9}}, derived from either of the two isomers (n-butane and isobutane) of butane.
The isomer n-butane can connect in two ways, giving rise to two "-butyl" groups:
- If it connects at one of the two terminal carbon atoms, it is normal butyl or n-butyl: {{chem2|\sCH2\sCH2\sCH2\sCH3}} (preferred IUPAC name: butyl)
- If it connects at one of the non-terminal (internal) carbon atoms, it is secondary butyl or sec-butyl: {{chem2|\sCH(CH3)\sCH2\sCH3}} (preferred IUPAC name: butan-2-yl)
The second isomer of butane, isobutane, can also connect in two ways, giving rise to two additional groups:
- If it connects at one of the three terminal carbons, it is isobutyl: {{chem2|\sCH2\sCH(CH3)2}} (preferred IUPAC name: 2-methylpropyl)
- If it connects at the central carbon, it is tertiary butyl, tert-butyl or t-butyl: {{chem2|\sC(CH3)3}} (preferred IUPAC name: tert-butyl)
Nomenclature
{{Unreferenced section|date=March 2025}}
According to IUPAC nomenclature, "isobutyl", "sec-butyl", and "tert-butyl" used to be allowed retained names. The latest guidance changed that: only tert-butyl is kept as preferred prefix, all other butyl-names are removed. In the convention of skeletal formulas, every line ending and line intersection specifies a carbon atom (unless otherwise indicated) saturated with single-linked hydrogen atoms (unless otherwise indicated). The "R" symbol indicates any radical or other non-specific functional group.
| class="wikitable"
!Skeletal formula of butyl !Preferred ! Alternate notation ! Fully systematic name ! Symbol |
| 100px
|n-butyl |butyl |butyl |butan-1-yl |Bu, n-Bu, nBu, nBu |
| 80px
|sec-butyl |butan-2-yl |1-methylpropyl |butan-2-yl |s-Bu, sBu, sBu |
| 80px
|isobutyl, iso-butyl |2-methylpropyl |2-methylpropyl |2-methylpropan-1-yl |i-Bu, iBu, iBu |
| 60px
|tert-butyl |tert-butyl |1,1-dimethylethyl |2-methylpropan-2-yl |t-Bu, tBu, tBu |
Butyl is the largest substituent for which trivial names are commonly used for all isomers.
The butyl group's carbon that is connected to the rest (R) of the molecule is called the RI or R-prime carbon. The prefixes sec (from "secondary") and tert (from "tertiary") refer to the number of additional side chains (or carbons) connected to the first butyl carbon. The prefix "iso" or "iso" means "isolated" while the prefix
Butan-2-yl (sec-butyl) group is chiral. The carbon atom at position 2 is a stereocenter. It has four different groups attached: {{chem2|\sH}}, {{chem2|\sCH3}}, {{chem2|\sCH2\sCH3}}, and {{chem2|\sR}} (the R group is not equal to those three groups). The names of the two chiral groups are: (2S)-butan-2-yl and (2R)-butan-2-yl.
Examples
The four isomers (ignoring stereoisomers) of "butyl acetate" demonstrate these four isomeric configurations. Here, the acetate radical appears in each of the positions where the "R" symbol is used in the chart above:
| File:Butylacetat.svg |
| {{center|n-butyl acetate}}
|{{center|sec-butyl acetate}} |{{center|isobutyl acetate}} |{{center|tert-butyl acetate}} |
sec-Butyl acetate is chiral, and has one stereocenter, and two enantiomers. The names of enantiomers are:
- [(2S)-butan-2-yl] acetate, (+)-sec-Butyl acetate
- [(2R)-butan-2-yl] acetate, (−)-sec-Butyl acetate
Therefore, for butyl acetate, the total number of isomers is five, if stereoisomers are included.
Etymology
Alkyl radicals are often considered as a series, a progression sequenced by the number of carbon atoms involved. In that progression, Butyl (containing 4 carbon atoms) is the fourth, and the last with preferred IUPAC name derived from its history. The word "butyl" is derived from butyric acid, a four-carbon carboxylic acid found in rancid butter. The name "butyric acid" comes from Latin butyrum, butter. Subsequent preferred IUPAC names for alkyl radicals in the series are simply named from the Greek number that indicates the number of carbon atoms in the group: pentyl, hexyl, heptyl, etc.
''tert''-Butyl "effects"
tert-Butyl group is special because it is bulky and it lacks alpha-H atoms. One way to assess the bulkiness of tert-Butyl uses the concept of ligand cone angle.{{cite journal | author = Tolman, C. A. | title = Steric Effects of Phosphorus Ligands in Organometallic Chemistry and Homogeneous Catalysis | journal = Chem. Rev. | year = 1977 | volume = 77 | pages = 313–48 | doi = 10.1021/cr60307a002 | issue = 3}}
| class="wikitable floatleft"
|+Cone angles of common phosphine ligands | |
| Ligand | Angle (°) |
|---|---|
| PH3 | 87 |
| P(CH3)3 | 118 |
| P(CH2CH3)3 | 132 |
| P(C6H5)3 | 145 |
| P(cyclo-C6H11)3 | 179 |
| P(t-Bu)3 | 182 |
The bulkiness of the tert-butyl substituent is used in chemistry for kinetic stabilization. The effect of the tert-butyl group on cyclization reactions is called the Thorpe–Ingold effect.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}}
The tert-butyl effect is an example of steric hindrance.
image: (t-BuO)2CrO2.png is a rare example of a stable chromate ester.]]
tert-Butyl groups lack alpha-hydrogen atoms. This aspect stabilizes di-tert-butyl chromate. Otherwise alpha-H's are abstracted by Cr(VI).{{Citation|last=Freeman|first=Fillmore|date=2001-04-15|publisher=John Wiley & Sons, Ltd|language=en|doi=10.1002/047084289x.rd059m|isbn=978-0471936237|title=Encyclopedia of Reagents for Organic Synthesis|chapter=Di-tert-butyl Chromate}}{{cite journal |doi=10.1139/v75-438|title=Esters Chromiques Dérivés d'Alcools Tertiaires|year=1975|last1=Richer|first1=Jean-Claude|last2=Hachey|first2=Jean-Marie|journal=Canadian Journal of Chemistry|volume=53|issue=20|pages=3087–3093|doi-access=}}
=Protection and deprotection=
A tert-butyl (tBu) ether is an acid-labile protecting group for alcohols.{{cite book|last1=Wuts|first1=P. G. M.|last2=Greene, T.W.|year=2006|title=Greene's Protective Groups in Organic Synthesis|doi=10.1002/0470053488|isbn= 9780470053485|publisher=J. Wiley|location=NY}}
A traditional way to introduce the tBu group to a hydroxyl group is by treating the compound with isobutylene in the presence of a Brønsted acid or Lewis acid.{{cite journal |last1=Micheli |first1=Robert A. |last2=Hojos |first2=Zoltan G. |last3=Cohen |first3=Noal |last4=Parrish |first4=David R. |last5=Portland |first5=Louis A. |last6=Sciamanna |first6=Werner |last7=Scott |first7=Melinda A. |last8=Wehrli |first8=Pius A. |title=Total syntheses of optically active 19-nor steroids. (+)-Estr-4-ene-3,17-dione and (+)-13.beta.-ethylgon-4-ene-3,17-dione |journal=The Journal of Organic Chemistry |date=March 1975 |volume=40 |issue=6 |pages=675–681 |doi=10.1021/jo00894a003 |url=https://doi.org/10.1021/jo00894a003 |language=en |issn=0022-3263}}{{cite journal |title=Proceedings of the Chemical Society. July 1961 |journal=Proceedings of the Chemical Society |date=1 January 1961 |issue=July |pages=249 |doi=10.1039/PS9610000229 |url=https://doi.org/10.1039/PS9610000229 |language=en |issn=0369-8718}}
Various acids can be used to cleave the tBu group, including both Brønsted acids such as trifluoroacetic acid and Lewis acids such as titanium tetrachloride.