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Pentyl group
Pentyl group
Pentyl group
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Pentyl group
Skeletal formula of pentyl with all explicit hydrogens added
Skeletal formula of pentyl with all explicit hydrogens added
Identifiers
3D model (JSmol)
ChemSpider
  • InChI=1S/C5H11/c1-3-5-4-2/h1,3-5H2,2H3 checkY
    Key: BFKVXNPJXXJUGQ-UHFFFAOYSA-N checkY
  • [CH2]CCCC
Properties
−C5H11
Molar mass 71.1408 g/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Pentyl is a five-carbon alkyl group or substituent with chemical formula −C5H11. It is the substituent form of the alkane pentane.

In older literature, the common non-systematic name amyl was often used for the pentyl group. Conversely, the name pentyl was used for several five-carbon branched alkyl groups, distinguished by various prefixes. The nomenclature has now reversed, with "amyl" being more often used to refer to the terminally branched group also called isopentyl, as in amobarbital.

A cyclopentyl group is a ring with the formula −C5H9.

The name is also used for the pentyl radical, a pentyl group as an isolated molecule. This free radical is only observed in extreme conditions.[1] Its formula is often written "C
5H
11•" or "• ⁠C
5H
11" to indicate that it has one unsatisfied valence bond. Radicals like pentyl are reactive, they react with neighboring atoms or molecules (like oxygen, water, etc.)

Older "pentyl" groups

[edit]

The following names are still used sometimes:

Name Structure IUPAC status IUPAC recomm. Examples
n-pentyl, amyl H
3C−(CH
2)
4− 		pentyl [2]
tert-pentyl H
3C−CH
2−(H
3C−)
2C− 		No longer recommended[3] 2-methylbutan-2-yl (aka 1,1-dimethylpropyl)[3] [4]
neopentyl (H
3C−)
3C−CH
2− 		No longer recommended[3] 2,2-dimethylpropyl[3] [5]
isopentyl, amyl, isoamyl (H
3C−)
2CH−(CH
2)
2− 		No longer recommended[3] 3-methylbutyl[3] [6]
sec-pentyl H
3C−(CH
2)
2−(H
3C−)CH− 		pentan-2-yl[7](or (1-methylbutyl)) [8]
3-pentyl (H
3C−CH
2−)
2CH− 		pentan-3-yl (also known as 1-ethylpropyl) [9]
sec-isopentyl (H
3C−)
2CH−(H
3C−)CH− 		3-methylbutan-2-yl (or 1,2-dimethylpropyl)
active pentyl H
3C−CH
2−(H
3C−)CH−CH
2− 		2-methylbutyl

Pentyl radical

[edit]

The free radical pentyl was studied by J. Pacansky and A. Gutierrez in 1983.[1] The radical was obtained by exposing bishexanoyl peroxide trapped in frozen argon to ultraviolet light, that caused its decomposition into two carbon dioxide (CO
2) molecules and two pentyl radicals.

Examples

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Pentyl group

View on Grokipedia
from Grokipedia
The pentyl group is a linear or branched derived from the saturated (C₅H₁₂) by the removal of a single , resulting in the general molecular C₅H₁₁–. As a monovalent group, it serves as a fundamental building block in , commonly attached to other functional groups to form larger molecules such as alcohols, ethers, and esters. The pentyl group exhibits eight constitutional isomers, each distinguished by the arrangement of its carbon skeleton and the position of the free valence (attachment point). These isomers arise from the possible structural variations of a five-carbon chain, including straight-chain and branched configurations, with primary, secondary, or tertiary attachment points. The systematic IUPAC designates them based on the parent , such as pentan-1-yl for the unbranched form. Key examples include:
Isomer Name (Common/Systematic)Structure (Condensed Formula)Description
n-Pentyl (pentan-1-yl)–CH₂CH₂CH₂CH₂CH₃Straight-chain primary .
Isopentyl (3-methylbutyl)–CH₂CH₂CH(CH₃)₂Primary group with branching at the third carbon.
Neopentyl (2,2-dimethylpropyl)–CH₂C(CH₃)₃Highly branched primary group centered on a quaternary carbon.
sec-Pentyl (pentan-2-yl)–CH(CH₃)CH₂CH₂CH₃Secondary group attached at the second carbon of a chain.
3-Pentyl (pentan-3-yl)–CH(CH₂CH₃)₂Secondary group with symmetric ethyl branches.
tert-Pentyl (2-methylbutan-2-yl)–C(CH₃)₂CH₂CH₃Tertiary group with two methyl branches.
2-Methylbutyl–CH₂CH(CH₃)CH₂CH₃Primary group with methyl branch at the second carbon.
3-Methylbutan-2-yl–CH(CH₃)CH(CH₃)CH₃Secondary group with adjacent methyl branches.
These structures reflect the diversity possible within the C₅H₁₁– formula, and their properties, such as boiling points and reactivity, vary accordingly due to differences in branching and steric hindrance. In and natural products, pentyl groups contribute to the and conformational flexibility of molecules, influencing and biological interactions. For instance, the isopentyl group is a key component in isopentyl , a volatile responsible for the characteristic banana-like aroma in certain fruits and flavorings. Similarly, n-pentyl derivatives appear in industrial applications, such as lubricants and additives, where the chain length enhances .

Definition and Nomenclature

Definition

In , an is a univalent derived from an by the removal of one from any carbon atom, resulting in the general CnH2n+1C_nH_{2n+1}. These groups serve as building blocks in larger organic molecules, where the alkyl moiety contributes hydrophobic character and influences molecular properties. The pentyl group specifically refers to the five-carbon alkyl fragment C5H11−C_5H_{11}-, obtained by removing one hydrogen atom from pentane (C5H12C_5H_{12}). Its molecular weight is 71.14 g/mol. In structural formulas, the straight-chain form of the pentyl group is commonly represented as CH3_3(CH2_2)3_3CH2_2-. As a representative example of a pentyl alkyl group, it exemplifies how longer-chain substituents extend the carbon skeleton in alkanes beyond simpler methyl (C1C_1) or ethyl (C2C_2) groups.

Nomenclature

The pentyl group is named in accordance with the International Union of Pure and Applied Chemistry (IUPAC) substitutive nomenclature system, where the unbranched five-carbon alkyl substituent derived from by removal of a is designated as pentyl or systematically as pentan-1-yl (CH₃(CH₂)₃CH₂–). Branched isomers employ descriptive prefixes and locants to indicate the position of branching; for example, the group (CH₃)₂CHCH₂CH₂– is named 3-methylbutyl (retained as isopentyl for general use), while (CH₃)₂CHCH(CH₃)– is 3-methylbutan-2-yl. Other variants include 2,2-dimethylpropyl (neopentyl) and 2-methylbutan-2-yl (tert-pentyl). These names are formed by replacing the final "-ane" of the parent with "-yl" to denote the monovalent radical, ensuring unambiguous structural description in chemical formulas. In historical nomenclature, particularly from 19th-century , trivial names derived from fusel oil (a of alcohol ) were commonly applied to pentyl groups and their derivatives. The term amyl originally referred to the normal or unbranched pentyl group (n-amyl or pentan-1-yl), but it frequently denoted mixtures or specifically the branched isoamyl (3-methylbutyl) due to its prevalence in natural sources like products. Additional terms included active amyl for the chiral pentan-2-yl or 2-methylbutyl (from 2-methylbutan-1-ol, noted for optical activity), and secondary amyl for pentan-3-yl. These names persisted in early industrial and pharmaceutical contexts, such as and amyl acetate, often leading to ambiguity without structural specification. The transition to systematic IUPAC naming accelerated in the to resolve such inconsistencies, with the 1979 IUPAC Recommendations on the Nomenclature of Organic Chemistry formally endorsing substitutive names like pentyl over trivial ones like amyl for precision in scientific communication. Retained trivial names such as isopentyl were permitted only for general or unmodified structures, while complex substituents required full systematic designation. This shift, building on earlier guidelines from the 1892 Geneva Congress, emphasized the longest chain and lowest locants for clarity. A key distinction exists between substituent nomenclature and parent hydride naming: pentyl denotes the group when attached to another molecular entity (e.g., in pentyl chloride), whereas pentane names the free hydrocarbon C₅H₁₂. This convention applies uniformly across alkyl series, preventing overlap in descriptive usage.

Structural Isomers

n-Pentyl group

The n-pentyl group, also known as the normal pentyl or straight-chain pentyl group, is the unbranched isomer of the pentyl substituent derived from n-pentane by removal of a hydrogen atom from a terminal carbon. Its IUPAC name is pentyl or, more systematically, pentan-1-yl. This group serves as a common alkyl substituent in organic synthesis and natural compounds, characterized by its linear arrangement of five carbon atoms. The of the n-pentyl group is \ceCH3−CH2−CH2−CH2−CH2−\ce{CH3-CH2-CH2-CH2-CH2-}, which can also be condensed as \ceCH3(CH2)4−\ce{CH3(CH2)4-} or represented with the general alkyl notation −\ceC5H11-\ce{C5H11}. In expanded form, it shows the first carbon as a (\ceCH3\ce{CH3}) bonded to a (\ce−CH2−\ce{-CH2-}), followed by three additional methylene groups, and terminating in another methylene for attachment (\ce−CH2−\ce{-CH2-}). The line-angle illustrates this as a continuous zigzag line of five bonds, with the free valence at one endpoint, emphasizing the saturated, acyclic hydrocarbon chain. Physical properties of compounds bearing the n-pentyl group are influenced by its nonpolar, hydrophobic nature, akin to the parent n-pentane (\ceC5H12\ce{C5H12}), which boils at 36.1 °C and density of 0.626 g/cm³ at 20 °C. n-Pentane is insoluble in (solubility <1 mg/mL at 20 °C) but readily soluble in nonpolar solvents like hexane and toluene due to similar van der Waals interactions.

Branched pentyl groups

The other seven constitutional isomers of the C₅H₁₁– alkyl substituent, excluding the straight-chain n-pentyl group, include both straight-chain secondary forms and various branched primary, secondary, and tertiary structures. These isomers arise from different positions of attachment and branching arrangements along the five-carbon framework. They are classified as primary, secondary, or tertiary based on the nature of the carbon atom at the point of attachment: primary groups have the free valence on a carbon bonded to only one other carbon (–CH₂–), secondary on a carbon bonded to two others (–CH–), and tertiary on a carbon bonded to three others (–C–). The primary isomers (straight-chain n-pentyl plus three branched) have the free valence at a terminal –CH₂– group, with the branched forms introducing variation through side chains. For example, the isopentyl group (3-methylbutyl) has the structure –CH₂CH₂CH(CH₃)₂, featuring branching at the third carbon from the attachment point. The 2-methylbutyl group is –CH₂CH(CH₃)CH₂CH₃, with branching at the second carbon. The neopentyl group (2,2-dimethylpropyl) is –CH₂C(CH₃)₃, notable for its highly symmetric, quaternary-like branching at the adjacent carbon, which results in significant steric hindrance despite being primary. Secondary isomers have the free valence at a –CH– carbon. The sec-pentyl group (pentan-2-yl) is CH₃–CH(–)CH₂CH₂CH₃, from a straight chain with attachment at the second carbon. The pentan-3-yl group is CH₃CH₂–CH(–)CH₂CH₃, symmetric with s on either side. The 3-methylbutan-2-yl group is CH₃–CH(–)CH(CH₃)CH₃, incorporating an isopropyl-like branch. The sole tertiary isomer is the tert-pentyl (2-methylbutan-2-yl), with structure (CH₃)₂C(–)CH₂CH₃, where the attachment carbon bears two methyl groups and one . Among these isomers, the isopentyl group is the most prevalent in natural compounds, appearing in metabolites such as , a key flavor component in fruits like bananas, and in prenylated biomolecules involved in microbial processes. This commonality stems from biosynthetic pathways favoring isoprenoid-like branching patterns.
Isomer NameIUPAC NameStructureClassification
Isopentyl3-Methylbutyl–CH₂CH₂CH(CH₃)₂Primary
2-Methylbutyl2-Methylbutyl–CH₂CH(CH₃)CH₂CH₃Primary
Neopentyl2,2-Dimethylpropyl–CH₂C(CH₃)₃Primary
sec-PentylPentan-2-ylCH₃–CH(–)CH₂CH₂CH₃Secondary
-Pentan-3-ylCH₃CH₂–CH(–)CH₂CH₃Secondary
-3-Methylbutan-2-ylCH₃–CH(–)CH(CH₃)CH₃Secondary
tert-Pentyl2-Methylbutan-2-yl(CH₃)₂C(–)CH₂CH₃Tertiary

Chemical Properties

Reactivity as a substituent

The pentyl group serves as a nonpolar, hydrophobic that enhances the of organic molecules, thereby influencing their in nonpolar solvents and increasing points relative to shorter alkyl analogs. For instance, in structures, the pentyl chain contributes significantly to overall hydrophobicity, promoting partitioning into environments. Similarly, longer alkyl substituents like pentyl raise points through enhanced van der Waals interactions, as observed in of alkanes and derivatives where each additional CH₂ unit elevates the by approximately 20–30°C. This effect is evident in esters such as pentyl , where the pentyl moiety increases compared to , aiding in organic phases during extractions. As a , the pentyl group exhibits considerable inertness in many , remaining stable under basic conditions due to the lack of acidic protons or electrophilic sites on the alkyl chain itself. Primary n-pentyl groups, in particular, tolerate strong bases without undergoing elimination or substitution at the chain unless activated by a . However, the chain is susceptible to oxidation under harsh conditions, such as with , which can cleave aliphatic C-C bonds to carboxylic acids, though this requires elevated temperatures and is less selective than for allylic positions. of the pentyl chain proceeds via free radical mechanisms with Cl₂ or Br₂ under or , preferentially at secondary carbons to form polyhalogenated products, demonstrating the group's vulnerability to radical processes despite its general stability. Representative reactions highlight the pentyl group's role in substituent contexts. In pentyl esters like , hydrolysis under acidic or basic conditions cleaves the ester bond to yield pentanol and the , with the pentyl chain acting as a spectator that modulates reaction rates through solvophobic effects but does not participate directly. For elimination, 1-bromopentane undergoes E2 reaction with a strong base like ethoxide in , forming 1-pentene via anti-periplanar β-hydrogen abstraction, illustrating how the linear chain facilitates concerted elimination without significant steric interference. Steric effects vary between isomers: the n-pentyl group imposes minimal hindrance in nucleophilic substitutions or additions due to its linear conformation, allowing efficient backside attack in SN2 reactions. In contrast, branched pentyl groups, such as isopentyl ((CH₃)₂CHCH₂CH₂-), introduce moderate steric bulk near the attachment point, slightly slowing SN2 rates compared to n-pentyl analogs in primary alkyl halides, though still less impeded than highly branched systems like neopentyl. This difference affects selectivity in reactions like esterifications, where branched forms may favor alternative pathways due to reduced accessibility.

Pentyl radical

The pentyl radical, denoted as C₅H₁₁•, is a reactive alkyl radical serving as a key intermediate in free radical chemistry, characterized by an on a carbon atom within a five-carbon chain. It exists in various forms depending on the position of the radical center, with the n-pentyl radical (CH₃(CH₂)₃CH₂•) being a primary radical and the sec-pentyl radical, such as the 2-pentyl isomer (CH₃CH₂CH₂CH•CH₃), being secondary. These isomers exhibit differing stabilities, with secondary pentyl radicals being more stable than primary ones primarily due to enhanced from adjacent C-H bonds, which delocalizes the unpaired electron. Formation of the pentyl radical typically occurs through homolytic cleavage of a C-H bond in or related derivatives, requiring significant input such as or photolysis. For instance, the 1-pentyl radical can be generated by of 1-iodopentane via C-I bond homolysis: C₅H₁₁–I → C₅H₁₁• + I•. The bond dissociation energy (BDE) for the relevant C-H bonds in n-pentane is approximately 413 kJ/mol for primary positions and 398 kJ/mol for secondary positions, reflecting the barrier to radical formation and contributing to their transient . In terms of reactivity, pentyl radicals participate in characteristic free radical processes, including addition to alkenes to form larger alkyl radicals, hydrogen abstraction from other molecules to yield or substituted products, and dimerization via coupling of two radicals to form C₁₀H₂₂. These reactions occur rapidly due to the high reactivity of the , with pentyl radicals often undergoing β-scission or at elevated temperatures (e.g., above 1000 K) to produce smaller alkenes and alkyl fragments. In polymerization contexts, pentyl radicals and similar alkyl species initiate radical chain reactions in the synthesis of polyolefins, such as , by adding to olefin monomers and propagating the chain growth.

Applications and Examples

Synthetic uses

The pentyl group, particularly the n-pentyl variant, finds significant application in the synthesis of esters used as solvents and flavoring agents. n-Pentyl acetate serves as an effective solvent in the production of paints, coatings, and adhesives due to its favorable solubility properties in organic media. Similarly, isoamyl acetate (derived from the isopentyl group) is widely employed in fragrances and food flavorings to impart a characteristic banana-like aroma, enhancing fruity profiles in perfumes and confectionery products. In pharmaceutical synthesis, pentyl chains contribute to the structure of intermediates like , a compound historically used as a and in experimental models to induce seizures for evaluating therapies. This GABA_A receptor antagonist has been investigated for its role in respiratory and cardiovascular stimulation, though its clinical use has diminished since the 1980s. Pentyl side chains are incorporated into copolymers to enhance mechanical flexibility and processability, particularly in materials like liquid-crystalline polymers where they influence transitional properties and chain dynamics. For instance, in conjugated polymers for organic solar cells, pentyl substituents improve and film morphology without halogenated solvents, thereby supporting better device performance through optimized side-chain engineering. In routes, pentyl halides act as alkylating agents in reactions, enabling the introduction of the pentyl moiety into various scaffolds, such as in the Friedel-Crafts of aromatics. Complementing this, pentylmagnesium , a , facilitates carbon-carbon bond formation through conjugate additions to α,β-unsaturated carbonyls and reactions with epoxides, providing access to extended alkyl chains in analogs and pharmaceuticals. These leverage the pentyl group's reactivity to build complex molecules efficiently in and industrial settings.

Natural occurrences

The pentyl group, particularly in its isoamyl (3-methylbutyl) form, occurs naturally as part of in fusel oil, a of alcoholic processes in such as . Fusel oil consists of higher alcohols including , which arises from the of branched-chain like during fermentation. This compound contributes to the characteristic aromas in fermented beverages and is a key component in natural essential oil-like mixtures derived from microbial activity. Biologically, pentyl groups are integral to short-chain alcohols involved in , such as pentanol isomers produced via the Ehrlich pathway during breakdown. In microbial and eukaryotic systems, the degradation of like yields and related pentyl-containing alcohols, which play roles in aroma formation and cellular signaling during and catabolic processes. These metabolites support energy production and stress responses in organisms ranging from to higher .

References

  1. https://openstax.org/books/organic-chemistry/pages/3-3-alkyl-groups
  2. https://pubchem.ncbi.nlm.nih.gov/compound/Isoamyl-Acetate
  3. https://www.sciencedirect.com/topics/chemistry/pentyl-group
  4. https://goldbook.iupac.org/terms/view/A00228
  5. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/03:_Organic_Compounds-_Alkanes_and_Their_Stereochemistry/3.03:_Alkyl_Groups
  6. https://iupac.qmul.ac.uk/BlueBook/P1.html
  7. https://pubchem.ncbi.nlm.nih.gov/compound/123167
  8. https://iupac.org/wp-content/uploads/2021/12/Principles_Leigh2011-compressed.pdf
  9. https://pubs.acs.org/doi/10.1021/ed073p1127
  10. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/amyl-alcohol
  11. https://iupac.qmul.ac.uk/BlueBook/PDF/P1.pdf
  12. https://docbrown.info/page06/isomers/isom-c5h11x.htm
  13. https://www.sciencedirect.com/topics/chemistry/isopentyl-group
  14. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_I_%28Liu%29/09%253A_Free_Radical_Substitution_Reaction_of_Alkanes/9.03%253A_Stability_of_Alkyl_Radicals
  15. https://pubs.acs.org/doi/10.1021/jp8020003
  16. https://www.webqc.org/compound-Pentane-C5H12.html
  17. https://pubs.acs.org/doi/10.1021/j100906a600
  18. https://www.sciencedirect.com/science/article/abs/pii/S154074891200079X
  19. https://pubs.acs.org/doi/10.1021/jp101159h
  20. https://www.sigmaaldrich.com/TR/en/product/sial/46022
  21. https://www.perfumersworld.com/view.php?pro_id=9FD00245
  22. https://pubchem.ncbi.nlm.nih.gov/compound/Pentylenetetrazole
  23. https://pubs.acs.org/doi/10.1021/ma00067a013
  24. https://www.researchgate.net/publication/359931225_Pentyl_side_chain-based_benzoditiophene_p-conjugated_polymer_for_non-halogenated_solvent_processed_organic_solar_cells
  25. https://pubs.acs.org/doi/10.1021/acs.iecr.5b02021
  26. https://pubs.acs.org/doi/10.1021/ol052068v
  27. https://pmc.ncbi.nlm.nih.gov/articles/PMC12300360/
  28. https://pmc.ncbi.nlm.nih.gov/articles/PMC10095983/
  29. https://pmc.ncbi.nlm.nih.gov/articles/PMC2804790/
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