IUPAC system and IUPAC rules of naming of halogen derivative of carboxylic acid

Functional class name = acyl or acid halide Substituent suffix = -oyl halide

• Acyl or acid halides are derivatives of carboxylic acids.
• The root name is based on the longest chain including the carbonyl group of the acyl group.
  
• Since the acyl group is at the end of the chain, the C=O carbon must be C1.
• The acyl halide suffix is appended after the hydrocarbon suffix minus the "e" : e.g. -ane + -oyl halide = -anoyl halide etc.
• The most common halide encountered is the chloride, hence acyl or acid chlorides, e.g. ethanoyl chloride

Functional group is an acyl halide therefore suffix = -oyl chloride

Hydrocarbon structure is an alkane therefore -an-
The longest continuous chain is C2 therefore root =eth

CH₃C(=O)Cl

ethanoyl chloride

Functional group is an acyl halide therefore suffix = -oyl chloride
Hydrocarbon structure is an alkane therefore -an-
The longest continuous chain is C4 therefore root =but

CH₃CH₂CH₂C(=O)Cl

butanoyl chloride

• 
  

Functional group is an acyl halide therefore suffix = -oyl chloride
Hydrocarbon structure is an alkane therefore -an-
The longest continuous chain is C3 therefore root =prop
The branch is a C1 alkyl group i.e. a methyl group
The functional groups requires numbering from theright as drawn, the substituent locant is 2-

(CH₃)₂CHC(=O)Cl

2-methylpropanoyl chloride