In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure R−CH2−C6H5. Benzyl features a benzene ring (C6H6) attached to a methylene group (−CH2−). [1]
In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure R−CH2−C6H5. Benzyl features a benzene ring (C6H6) attached to a methylene group (−CH2−). [1]
In IUPAC nomenclature, the prefix benzyl refers to a C6H5CH2 substituent, for example benzyl chloride or benzyl benzoate. Benzyl is not to be confused with phenyl with the formula C6H5. The term benzylic is used to describe the position of the first carbon bonded to a benzene or other aromatic ring. For example, (C6H5)(CH3)2C+ is referred to as a "benzylic" carbocation. The benzyl free radical has the formula C6H5CH2•. The benzyl cation or phenylcarbenium ion is the carbocation with formula C6H5CH+2; the benzyl anion or phenylmethanide ion is the carbanion with the formula C6H5CH−2. None of these species can be formed in significant amounts in the solution phase under normal conditions, but they are useful referents for discussion of reaction mechanisms and may exist as reactive intermediates.
Benzyl is most commonly abbreviated Bn. For example, benzyl alcohol can be represented as BnOH. Less common abbreviations are Bzl and Bz, the latter of which is ambiguous as it is also the standard abbreviation for the benzoyl group C6H5C(O)−. Likewise, benzyl should not be confused with the phenyl group C6H5−, abbreviated Ph.
The enhanced reactivity of benzylic positions is attributed to the low bond dissociation energy for benzylic C−H bonds. Specifically, the bond C6H5CH2−H is about 10–15% weaker than other kinds of C−H bonds. The neighboring aromatic ring stabilizes benzyl radicals. The data tabulated below compare benzylic C−H bond to related C−H bond strengths.
| Bond | Bond | Bond-dissociation energy [2] [3] | Comment | |
|---|---|---|---|---|
| (kcal/mol) | (kJ/mol) | |||
| C6H5CH2−H | benzylic C−H bond | 90 | 377 | akin to allylic C−H bonds such bonds show enhanced reactivity |
| H3C−H | methyl C−H bond | 105 | 439 | one of the strongest aliphatic C−H bonds |
| C2H5−H | ethyl C−H bond | 101 | 423 | slightly weaker than H3C−H |
| C6H5−H | phenyl C−H bond | 113 | 473 | comparable to vinyl radical, rare |
| CH2=CHCH2−H | allylic C–H bond | 89 | 372 | similar to benzylic C-H |
| (C6H4)2CH−H | fluorenyl C–H bond | 80 | more activated vs diphenylmethyle (pKa = 22.6) | |
| (C6H5)2CH−H | diphenylmethyl C–H bond | 82 | "doubly benzylic" (pKa = 32.2) | |
| (C6H5)3C−H | trityl C–H bond | 81 | 339 | "triply benzylic" |
The weakness of the C−H bond reflects the stability of the benzylic radical. For related reasons, benzylic substituents exhibit enhanced reactivity, as in oxidation, free radical halogenation, or hydrogenolysis. As a practical example, in the presence of suitable catalysts, p-xylene oxidizes exclusively at the benzylic positions to give terephthalic acid:
Millions of tonnes of terephthalic acid are produced annually by this method. [4]
In a few cases, these benzylic transformations occur under conditions suitable for lab synthesis. The Wohl-Ziegler reaction will brominate a benzylic C–H bond: (ArCHR2 → ArCBrR2). [5] Any non-tertiary benzylic alkyl group will be oxidized to a carboxyl group by aqueous potassium permanganate (KMnO4) or concentrated nitric acid (HNO3): (ArCHR2 → ArCOOH). [6] Finally, the complex of chromium trioxide and 3,5-dimethylpyrazole (CrO3−dmpyz) will selectively oxidize a benzylic methylene group to a carbonyl: (ArCH2R → ArC(O)R). [7] 2-iodoxybenzoic acid in DMSO performs similarly. [8]
Benzyl groups are occasionally employed as protecting groups in organic synthesis. Their installation and especially their removal require relatively harsh conditions, so benzyl is not typically preferred for protection. [9]
Benzyl is commonly used in organic synthesis as a robust protecting group for alcohols and carboxylic acids.
Benzyl ethers can be removed under reductive conditions, oxidative conditions, and the use of Lewis acids. [9]
p-Methoxybenzyl (PMB) is used as a protecting group for alcohols in organic synthesis (4-Methoxybenzylthiol is used to protect thiols).
The benzyl group is occasionally used as a protecting group for amines in organic synthesis. Other methods exist. [9]
In chemistry, an ester is a functional group derived from an acid in which the hydrogen atom (H) of at least one acidic hydroxyl group of that acid is replaced by an organyl group. Analogues derived from oxygen replaced by other chalcogens belong to the ester category as well. According to some authors, organyl derivatives of acidic hydrogen of other acids are esters as well, but not according to the IUPAC.
In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6H5, and is often represented by the symbol Ph. The phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen, which may be replaced by some other element or compound to serve as a functional group. A phenyl group has six carbon atoms bonded together in a hexagonal planar ring, five of which are bonded to individual hydrogen atoms, with the remaining carbon bonded to a substituent. Phenyl groups are commonplace in organic chemistry. Although often depicted with alternating double and single bonds, the phenyl group is chemically aromatic and has equal bond lengths between carbon atoms in the ring.
A protecting group or protective group is introduced into a molecule by chemical modification of a functional group to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis.
In organic chemistry, an allyl group is a substituent with the structural formula −CH2−HC=CH2. It consists of a methylene bridge attached to a vinyl group. The name is derived from the scientific name for garlic, Allium sativum. In 1844, Theodor Wertheim isolated an allyl derivative from garlic oil and named it "Schwefelallyl". The term allyl applies to many compounds related to H2C=CH−CH2, some of which are of practical or of everyday importance, for example, allyl chloride.
Ceric ammonium nitrate (CAN) is the inorganic compound with the formula (NH4)2[Ce(NO3)6]. This orange-red, water-soluble cerium salt is a specialised oxidizing agent in organic synthesis and a standard oxidant in quantitative analysis.
The Curtius rearrangement, first defined by Theodor Curtius in 1885, is the thermal decomposition of an acyl azide to an isocyanate with loss of nitrogen gas. The isocyanate then undergoes attack by a variety of nucleophiles such as water, alcohols and amines, to yield a primary amine, carbamate or urea derivative respectively. Several reviews have been published.
1,1'-Carbonyldiimidazole (CDI) is an organic compound with the molecular formula (C3H3N2)2CO. It is a white crystalline solid. It is often used for the coupling of amino acids for peptide synthesis and as a reagent in organic synthesis.
The Corey–Kim oxidation is an oxidation reaction used to synthesize aldehydes and ketones from primary and secondary alcohols. It is named for American chemist and Nobel Laureate Elias James Corey and Korean-American chemist Choung Un Kim.
In organic chemistry, benzoyl is the functional group with the formula −COC6H5 and structure −C(=O)−C6H5. It can be viewed as benzaldehyde missing one hydrogen. The benzoyl group has a mass of 105 amu.
Diphenylmethane is an organic compound with the formula (C6H5)2CH2 (often abbreviated CH
2Ph
2). The compound consists of methane wherein two hydrogen atoms are replaced by two phenyl groups. It is a white solid.
A boronic acid is an organic compound related to boric acid in which one of the three hydroxyl groups is replaced by an alkyl or aryl group. As a compound containing a carbon–boron bond, members of this class thus belong to the larger class of organoboranes.
The Achmatowicz reaction, also known as the Achmatowicz rearrangement, is an organic synthesis in which a furan is converted to a dihydropyran. In the original publication by the Polish chemist Osman Achmatowicz Jr. in 1971 furfuryl alcohol is reacted with bromine in methanol to 2,5-dimethoxy-2,5-dihydrofuran which rearranges to the dihydropyran with dilute sulfuric acid. Additional reaction steps, alcohol protection with methyl orthoformate and boron trifluoride) and then ketone reduction with sodium borohydride produce an intermediate from which many monosaccharides can be synthesised.
In organic synthesis, cyanation is the attachment or substitution of a cyanide group on various substrates. Such transformations are high-value because they generate C-C bonds. Furthermore nitriles are versatile functional groups.
In chemistry, hyponitrite may refer to the anion N
2O2−
2 ([ON=NO]2−), or to any ionic compound that contains it. In organic chemistry, it may also refer to the group −O−N=N−O−, or any organic compound with the generic formula R1−O−N=N−O−R2, where R1 and R2 are organic groups. Such compounds can be viewed as salts and esters of hyponitrous acid. An acid hyponitrite is an ionic compound with the anion HN
2O−
2 ([HON=NO]−).
Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters. The reaction mainly applies to primary and secondary alcohols. Secondary alcohols form ketones, while primary alcohols form aldehydes or carboxylic acids.
Diphenyldichloromethane is an organic compound with the formula (C6H5)2CCl2. It is a colorless solid that is used as a precursor to other organic compounds.
Trichloroacetonitrile is an organic compound with the formula CCl3CN. It is a colourless liquid, although commercial samples often are brownish. It is used commercially as a precursor to the fungicide etridiazole. It is prepared by dehydration of trichloroacetamide. As a bifunctional compound, trichloroacetonitrile can react at both the trichloromethyl and the nitrile group. The electron-withdrawing effect of the trichloromethyl group activates the nitrile group for nucleophilic additions. The high reactivity makes trichloroacetonitrile a versatile reagent, but also causes its susceptibility towards hydrolysis.
A photolabile protecting group is a chemical modification to a molecule that can be removed with light. PPGs enable high degrees of chemoselectivity as they allow researchers to control spatial, temporal and concentration variables with light. Control of these variables is valuable as it enables multiple PPG applications, including orthogonality in systems with multiple protecting groups. As the removal of a PPG does not require chemical reagents, the photocleavage of a PPG is often referred to as "traceless reagent processes", and is often used in biological model systems and multistep organic syntheses. Since their introduction in 1962, numerous PPGs have been developed and utilized in a variety of wide-ranging applications from protein science to photoresists. Due to the large number of reported protecting groups, PPGs are often categorized by their major functional group(s); three of the most common classifications are detailed below.
The Mukaiyama hydration is an organic reaction involving formal addition of an equivalent of water across an olefin by the action of catalytic bis(acetylacetonato)cobalt(II) complex, phenylsilane and atmospheric oxygen to produce an alcohol with Markovnikov selectivity.
The Stahl oxidation is a copper-catalyzed aerobic oxidation of primary and secondary alcohols to aldehydes and ketones. Known for its high selectivity and mild reaction conditions, the Stahl oxidation offers several advantages over classical alcohol oxidations.
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