Alkyl Halides: Classification and Nomenclature

Alkyl Halides

• Organic compounds with a halogen atom bonded to a carbon atom are known as Alkyl Halides

Classification of Halogenated Organic Compounds

• There are three major classes of halogenated organic compounds, these compounds are the alkyl halides, the vinyl halides, and the aryl halides.
• The halogen atom is bonded to one of the sp³ carbons of the alkyl group in alkyl halides.
• The halogen atom is bonded to one of the sp² carbons of the alkene or the carbons of the aromatic ring in vinyl halides and aryl halides
• Size increases from Fluorine -> Chlorine -> Bromine -> Iodine
• Electronegativity increases from Iodine -> Bromine -> Chlorine -> Fluorine

Classification of Alkyl Halides

• Alkyl halides can be classified as methyl halides, primary (1°) halides, secondary (2°) halides, and tertiary (3°) halides.
• Alkyl halides can also be classified as Vicinal dihalides and Geminal dihalides.

Nomenclature

• Find the longest carbon chain and name it as the parent.
• If a double or triple bond is present, the parent must contain it.
• Write the parent name using Prefix (no. of carbon) + Suffix (ane or other)
• Number the carbons of the parent chain, starting at the end nearer to the first substituent.
• Whether the substituent is alkyl or halogen.
• Write the substituent according to the alphabetical order before the parent name.
• Note that Halogen (X) is a substituent, not a functional group.

IUPAC and Common Names

• Haloalkanes is an IUPAC Name such as 2-bromo-2-methylpropane
• Alkyl halide is a common name such as tert-butyl bromide.
• Additional examples of IUPAC Name : Common name are:
  • 2-Iodo-2-methylbutane (tert. pentyl iodide)
  • 2-chlorobutane (Sec.butyl chloride)
  • chloroethene (vinyl chloride)
  • 3-chloroprop-1-ene (Allyl chloride)
  • 1-bromo-2-methylpropane (isobutyl bromide)
  • 2-bromobutane (sec.butyl bromide)
  • 2-bromo-2-methylpropane (tert.butyl bromide)

Preparation of Alkyl Halides

• The preparation of alkyl halides can be done from Alkanes, Alkenes, Alkynes and Alcohols.

From Alkanes

• Preparation of Alkyl Halides from Alkanes can be done By Free radical substitution (Halogenation: Bromination or chlorination)

From Alkenes

• Preparation of Alkyl Halides from Alkenes can be done using Hydrohalogenation
• Reaction with Br2/ CCl4 is also possible via Halogenation with resulting Alkyl halide
• Addition can occur in the presence of Allylic Halogenation with resulting Alkyl halide

From Alkynes

• Preparation of Alkyl Halides from Alkynes can be done with Hydrohalogenation
• Reaction with Br2/ CCl4 is also possible via Halogenation with resulting Alkyl halide

From Alcohols

• Preparation of Alkyl Halides from Alcohols can be done By using Hydrogen Halides (HI >HBr >HCl)
• The Lucas Test is used to differentiate between primary, secondary, and tertiary alcohols using a solution of anhydrous zinc chloride in concentrated hydrochloric acid.
• This solution is commonly referred to as the Lucas reagent.
• Also preparation can be done By using Halogenating agents, where These reactions do not proceed via carbocation formation; hence no rearrangement occurs.
• Summary of the Best Reagents for Converting Alcohols to Alkyl Halides:
  • In primary class alcohol- Chloride is SOC12, Bromide is PBr3 or HBr*, Iodide is P/I2
  • In secondary class alcohol- Chloride is SOCI2​, Bromide is PBr3*, Iodide is P/I2*
  • In tertiary class alcohol- Chloride is HCl, Bromide is HBr, Iodide is HI*

Alkyl Halides Physical Properties

• Alkyl halides have higher molecular weight than the corresponding alkanes so they have higher boiling points.
• For a given alkyl group (R), The boiling point increases as the atomic weight of the halogen X increases
• The boiling point for RI > RBr > RCI > RF; i.e., for example; CH3CH2I > CH3CH2Br > CH3CH2Cl > CH3CH2F
• Alkyl halides are insoluble in water despite the polar C-X bond.
• The attraction between the R-X molecules is stronger than the alkyl halide and water.
• Alkyl halides have good solubility in nonpolar solvents.

Polarity of the C-X Bond

• The carbon-halogen bond of alkyl halides is polarized due to the difference in electronegativity.
• The carbon atom bears a partial positive charge, and the halogen atom has a partial negative charge.

Reactions of Alkyl Halides

• Reactions that Alkyl Halides take part in are Elimination and Nucleophilic Substitution.

Elimination Reactions

• Produce double bonds.
• The alkyl halides lose hydrogen and the halide.
• Also called dehydrohalogenation (-HX).
• Occurs via El vs E2 mechanisms.

Nucleophilic substitution reactions

• The halogen atom is replaced with a nucleophile.
• The c-X bond breaks heterolytically and X- leaves via SN1 or SN2 mechanisms

SN1 reaction

• For SN1 Reaction the L.G. leaves before the Nu attack

SN2 reaction

• For SN2 Reaction the L.G. leaves with the Nu attack

Factors affecting the SN mechanism

• Unimolecular nucleophilic substitution (SN1)
  • Two Steps
  • Rearranged products
  • Stability 3° > 2° R
  • Good leaving group is I-> Br -> Cl-
  • Weak nucleophile (may also be solvent C2H5OH, CH3OH, H2O, NH3, RNH2
  • Polar Protic solvent (solvents contain hydrogen bond donor (HBD) is C2H5OH, CH3OH, H2O
• Bimolecular nucleophilic substitution SN2
  • ONe step
  • No Rearranged products
  • CH,X > 1° > 2° R
  • Poor leaving group -OH >F- > Cl- > Br -> I-
  • Strong nucleophile -I- Br- Cl--CN-SH-N3 (NaN3) RS- -OH (NaOH), -OCH3 (NaOCH3), -OC2H5 (NaOEt)
  • Polar Aprotic (polar solvents lacking HBD) or nonpolar solvents
• SN2 reactions are beneficial in organic synthesis because they allow converting one functional group into another

Factors That Enhance Elimination Reactions

• Alkyl groups: Tertiary alkyl group.
• Strong Bulky (large) bases.
• Solvent: Polar protic Solvent only e.g., alcohol.
• Conditions of reaction: High temperature

Competition Between Elimination and Nucleophilic Substitution

• Strong & bulky bases favor Elimination product:
  • -NH2 & tert-butoxide (NaNH, & (CH₃)₂CO-+K