Skip to main content
Library homepage
 
Chemistry LibreTexts

Alpha Halogenation

  • Page ID
    1157
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    A carbonyl containing compound with \(\alpha\) hydrogens can undergo a substitution reaction with halogens. This reaction comes about because of the tendency of carbonyl compounds to form enolates in basic condition and enols in acidic condition. In these cases even weak bases, such as the hydroxide anion, is sufficient enough to cause the reaction to occur because it is not necessary for a complete conversion to the enolate. For this reaction Cl2, Br2 or I2 can be used as the halogens.

    General reaction

    Figure 1.jpg

    Example

    Figure 2.jpg

    Acid Catalyzed Mechanism

    Under acidic conditions the reaction occurs thought the formation of an enol which then reacts with the halogen.

    Step 1: Protonation of the carbonyl

    Figure 3.jpg

    Step 2: Enol formation

    Figure 4.jpg

    Step 3: SN2 attack

    Fgure 5.jpg

    Step 4: Deprotonation

    Figure 6.jpg

    Base Catalyzed Mechanism

    Under basic conditions the enolate forms and then reacts with the halogen. Note! This is base promoted and not base catalyzed because an entire equivalent of base is required.

    Step 1: Enolate formation

    Figure 7.jpg

    Step 2: SN2 attack

    Figure 8.jpg

    Overreaction during base promoted α halogenation

    The fact that an electronegative halogen is placed on an α carbon means that the product of a base promoted α halogenation is actually more reactive than the starting material. The electron withdrawing effect of the halogen makes the α carbon even more acidic and therefor promotes further reaction. Because of this multiple halogenations can occur. This effect is exploited in the haloform reaction discussed later. If a monohalo product is required then acidic conditions are usually used.

    Figure 9.jpg

    The Haloform Reaction

    Methyl ketones typically undergo halogenation three times to give a trihalo ketone due to the increased reactivity of the halogenated product as discussed above. This trihalomethyl group is an effective leaving group due to the three electron withdrawing halogens and can be cleaved by a hydroxide anion to effect the haloform reaction. The product of this reaction is a carboxylate and a haloform molecule (CHCl3, CHBr3, CHI3). Overall the haloform reaction represents an effective method for the conversion of methyl ketones to carboxylic acids. Typically, this reaction is performed using iodine because the subsequent iodoform (CHI3) is a bright yellow precipitate which is easily filtered off.

    General reaction

    Figure 10.jpg

    Example: The Haloform Reaction

    Figure 11.jpg

    Mechanism

    1) Formation of the trihalo species

    Figure 12.jpg

    2) Nulceophilic attack on the carbonyl carbon

    Figure 13.jpg

    3) Removal of the leaving group

    Figure 14.jpg

    4) Deprotonation

    Figure 15.jpg

    Exercise \(\PageIndex{1}\)

    Please draw the products of the following reactions

    Figure 16.jpg

    Answer

    166152577467808613.png


    Alpha Halogenation is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Steven Farmer.

    • Was this article helpful?