haloalkanes and haloarenes

haloalkanes/ alkyl halides

• the replacement of H atoms from a hydrocarbon, by halogens (F, Cl, Br, I) forms a haloalkane

• the X atom is attached to a sp3 hybridised C-atom

haloarenes

• the replacement of H atoms from a benzene ring, with a halogen (F, Cl, Br, I) forms a haloarene

• the X atom is attached to a sp2 hybridised C-atom

classification of haloalkanes/ arenes on the basis on no. of X atoms

• mono: has 1 X atom

• di: has 2 X atoms

• tri: has 3 X atoms

classification haloalkanes in terms of the C—X bond

• alkyl halides:

  • the X atom is bonded to an alkyl group R

  • they form a homologous series CnH2n+1X

    • primary: the C atom is attached to 1 alkyl group

    • secondary: the C atom is attached to 2 alkyl groups

    • tertiary: the C atom is attached to 3 alkyl groups

• allylic halides:

  • the X atom is bonded to a C atom which is bonded to another C atom which is double bonded to anotherr C atom

• benzyllic halides:

  • the X atom is bonded to a C atom which is bonded to benzene ring

classification of haloarenes in terms of the C—X bond

• vinylic halides:

  • in these, the X atom is bonded to the sp2 hybridised C atom (the one with a double bond

  • eg: halocycohexane

• aryl halides:

  • the X atom is directly bonded to an aromatic ring

  • eg: halobenzene

nomenclature for mono-substituted haloalkane

• common name: n-/ iso-/ sec-/ tert- + alkyl group name + X-ide

  • eg: n-propyl iodide

• IUPAC: position + Xo + alkane

  • eg: 1-iodo propane

how to use the prefixes

• n → straight chain

• iso → branch on second carbon

• sec → func. group on secondary carbon (a C atom attached to 2 other C atoms)

• tert → func. group on tertiary carbon (a C atom attached to 3 other C atoms)

• neo → highly branched C atom (C atom is attached to a C atom which is attached to 3 other C atoms)

nomenclature for disubstituted haloalkane

first, there are 2 types

• gem halides: the 2 X atoms are on the same C atom

• vic halides: the 2 X atoms are on adjacent C atoms

• common name:

  • gem halides → alkylidene halides

  • vic halides → alkylene dihalides

• IUPAC: dihaloalkanes

nomenclature for haloarenes

• mono-substituted: halobenzene for both common and iupac names

• disubstituted:

  • common name: o-/m-/p- + di + halobenzene

  • iupac: 1,2/1,3/1,4 + di + halobenzene

nature of the C—X bond

• electronegativity of X is more than C

• therefore, the shared pair electron is closer to the X atom

• this results in X atom bearing a partial -ve charge and C atom bearing a partial +ve charge

  • this implies C—X bond is polarised

polarity of C—X bond decreases as- F > Cl > Br > I (electronegativity decreases)

length of C—X bond decreases as- I > Br > Cl > F (atom size decreases → bond length decreases)