Why is methyl chloride a primary alkyl halide?


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> In a primary (1°) halogenoalkane, the carbon which carries the halogen atom is only attached to one other alkyl group.  
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> [Chemguide.co.uk](http://www.chemguide.co.uk/organicprops/haloalkanes/background.html)
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For $\ce{CH3Cl}$, the carbon which carries the halogen atom has no other alkyl groups attached, yet my class notes say "the alkyl halide has a primary centre". So is a primary alkyl group one with is attached to **0** or **1** other alkyl group and not just **1**?




Yes, you are correct. 


The IUPAC defines [alkyl groups](http://goldbook.iupac.org/A00228.html) in the following manner:



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> Univalent groups derived from alkanes by removal of a hydrogen atom from any carbon atom $\ce{–C\_{$n$}H\_{$2n+1$}}$. 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 $\ce{H(CH2)\_{$n$}}$. The groups $\ce{RCH2}$, $\ce{R2CH~(R \neq H)}$, and $\ce{R3C~(R \neq H)}$ are primary, secondary and tertiary alkyl groups, respectively.
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Note that for primary alkyl groups hydrogen may be a remainder, i.e. $\ce{R $=$ H, Alkyl, Aryl}$, therefore any halogenoalkane with the structure $\ce{RH2C-X}$ is primary.


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