Acyclic Hydrocarbons

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Acyclic (branched or unbranched) and cyclic (with or without side chain) hydrocarbons having one or more carbon-carbon triple bonds. See also alkynes.


Compounds arising by replacement of one or both hydrogen atoms of acetylene (ethyne) by a metal or other cationic group. E.g. NaC[triple bond]CH monosodium acetylide. NOC Rule C-84.3. By extension, analogous compounds derived from terminal acetylenes, RC[triple bond]CH.

aliphatic compounds:

Acyclic or cyclic, saturated or unsaturated carbon compounds, excluding aromatic compounds.


Acyclic branched or unbranched hydrocarbons having the general formula CnH2n+2, and therefore consisting entirely of hydrogen atoms and saturated carbon atoms. NOC Rule A-1.1. Cf. cycloalkanes.

alkanium ions:

Carbocations derived from alkanes by C-hydronation containing at least one pentacoordinate carbon atom. RNRI Rule RC- Cf. carbonium ions. E.g. +CH5 methanium, [C2H7]+ ethanium.


Acyclic branched or unbranched hydrocarbons having one carbon-carbon double bond and the general formula CnH2n . Acyclic branched or unbranched hydrocarbons having more than one double bond are alkadienes, alkatrienes, etc. NOC Rule A-3.1. See also olefins.

alkyl groups:*

Univalent groups derived from alkanes by removal of a hydrogen atom from any carbon atom: CnH2n+1-. The groups derived by removal of a hydrogen atom from a terminal carbon atom of unbranched alkanes form a subclass of normal alkyl (n-alkyl) groups: H[CH2]n-. NOC Rule A-1.2. The groups RCH2-, R2CH- (R not equal to H), and R3C- (R not equal to H) are primary, secondary and tertiary alkyl groups respectively. See also cycloalkyl groups. Cf. hydrocarbyl groups.

alkyl radicals:*

Carbon-centered radicals derived formally by removal of one hydrogen atom from an alkane. RNRI Rule RC-81.1.1. E.g. CH3CH2CH2. propyl.


1. An old term, which is not recommended, for alkenes, especially those of low molecular weight.

2. An old term for alkanediyl groups commonly but not necessarily having the free valencies on adjacent carbon atoms. E.g. -CH(CH3)CH2- propylene (systematically called propane-1,2-diyl).


Carbenes R2C: formed by mono or dialkyl substitution of methylene, H2C: RNRI Rule RC-81.1.3. E.g. CH3CH2CH: propylidene.

alkylidene groups:*

The divalent groups formed from alkanes by removal of two hydrogen atoms from the same carbon atom, the free valencies of which are part of a double bond. GNOC Recom. R-2.5. E.g. (CH3)2C= propan-2-ylidene.


Carbenes RC[3dot] formed by alkyl substitution of methyne, HC[3dot] RNRI Rule RC-81.1.3. E.g. CH3CH2C[3dot] propylidyne.


Acyclic branched or unbranched hydrocarbons having a carbon-carbon triple bond and the general formula CnH2n-2 , RC[triple bond]CR. Acyclic branched or unbranched hydrocarbons having more than one triple bond are known as alkadiynes, alkatriynes, etc. NOC Rule A-3.2. See also acetylenes.


Hydrocarbons (and by extension, derivatives formed by substitution) having two double bonds from one carbon atom to two others: R2C=C=CR2. (The simplest member, propadiene, is known as allene.) See also cumulenes, dienes.

allylic groups:

The group CH2=CHCH2- (allyl) and derivatives formed by substitution. The term 'allylic position' or 'allylic site' refers to the saturated carbon atom. A group, such as -OH, attached at an allylic site is sometimes described as 'allylic'.

allylic intermediates:

Carbanions, carbenium ions, or radicals, formally derived by detachment of one hydron, hydride or hydrogen from the CH3 group of propene or derivatives thereof. E.g. H2C=CHCH2+ allyl cation.


Hydrocarbons (and by extension, derivatives formed by substitution) having three or more cumulative double bonds. E.g. R2C=C=C=CR2. (Cumulative double bonds are those present in a chain in which at least three contiguous atoms are joined by double bonds.) NOC Rule A-21.1, footnote. See also allenes, heterocumulenes.


Compounds that contain two fixed double bonds (usually assumed to be between carbon atoms). See alkenes, olefins. Dienes in which the two double-bond units are linked by one single bond are termed conjugated, e.g. CH2=CH-CH=CH2 buta-1,3-diene. Dienes in which the double bonds are adjacent are called cumulative, e.g. CH3CH=C=CH2 buta-1,2-diene. See also allenes, cumulenes . Those in which one or more of the unsaturated carbon atoms is replaced by a heteroatom may be called heterodienes.


Compounds consisting of carbon and hydrogen only.


Acyclic and cyclic hydrocarbons having one or more carbon-carbon double bonds, apart from the formal ones in aromatic compounds. The class olefins subsumes alkenes and cycloalkenes and the corresponding polyenes. See also cycloalkanes.


Obsolescent term for saturated hydrocarbons, commonly but not necessarily acyclic. Still widely used in the petrochemical industry, where the term designates acyclic saturated hydrocarbons, and stands in contradistinction to naphthenes.

vinylic groups:

The vinyl group (CH2=CH-) and derivatives formed by substitution. Informally, a group, such as -OH, attached to the free valence of a (substituted) vinyl group is sometimes referred to as 'vinylic'.

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