{"id":5444,"date":"2012-08-07T17:10:21","date_gmt":"2012-08-07T17:10:21","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=5444"},"modified":"2024-10-23T10:35:40","modified_gmt":"2024-10-23T15:35:40","slug":"the-conjugate-acid-is-a-better-leaving-group","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2012\/08\/07\/the-conjugate-acid-is-a-better-leaving-group\/","title":{"rendered":"The Conjugate Acid Is A Better Leaving Group"},"content":{"rendered":"

Adding Acid Increases Leaving Group Ability<\/strong><\/p>\n

Sure, it’s one thing to recognize halogens (Cl, Br, and I) as having high leaving group ability, as well as tosylate (TsO- ) and mesylate (MsO-) but what if you have a functional group like HO- ? How do you get that pesky hydroxide to leave? Read on!<\/p>\n

\"Summary-Conjugate<\/a><\/p>\n

Table of Contents<\/strong><\/p>\n

    \n
  1. Weak Bases Are Good Leaving Groups – And The Hydroxide Ion (-OH) Is Not A Good Leaving Group<\/a><\/li>\n
  2. Adding Acid Converts The Leaving Group From The Strong Base (-OH) To The Much Weaker Base H2<\/sub>O<\/a><\/li>\n
  3. The Conjugate Acid Is Always A Better Leaving Group<\/a><\/li>\n
  4. Halogens Can Be Made Into Better Leaving Groups By Adding Lewis Acids Such As Silver Ion (Ag+)<\/a><\/li>\n
  5. Notes<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Weak Bases Are Good Leaving Groups – And The Hydroxide Ion (-OH) Is Not A Good Leaving Group<\/strong><\/h2>\n

    One of the key factors that determines whether a nucleophilic substitution reaction will happen or not is the identity of the leaving group. Previously, we’ve seen that good leaving groups are weak bases (See post: What Makes A Good Leaving Group<\/a><\/em><\/span>?)<\/p>\n

    That means that if you see a halogen (Cl, Br, or I) or a tosylate (OTs) or mesylate (OMs) on your molecule, these are all good candidates to be leaving groups, since Cl(\u2013), Br(\u2013), I(\u2013), TsO(\u2013), and MsO(\u2013) are all weak bases.<\/p>\n

    Here’s the problem. What if we have a functional group on a molecule that we’d like to involve in a substitution reaction, but it isn’t a good leaving group like the ones listed above? For instance, the reactions listed below don’t work<\/strong>, because the leaving group (HO-) is a strong base and thus a poor leaving group.<\/p>\n

    \"hydroxide<\/p>\n

    <\/a>2. Adding Acid Converts The Leaving Group From The Strong Base (-OH) To The Much Weaker Base H2<\/sub>O<\/h2>\n

    What can we do to involve OH (and other similar groups) so that they can participate in substitution reactions? Well, we need our leaving group to be a weaker base. How do we make it a weaker base? By removing some of its electron density. The best way to do this is to\u00a0treat it with acid. <\/strong>This will make the conjugate acid<\/strong>\u00a0of our leaving group, which will be a weaker base. <\/strong>And then these reactions will proceed nicely.<\/p>\n

    \"addition<\/p>\n

    Here’s an SN<\/sub>2 example:<\/p>\n

     <\/p>\n

    \"example<\/h2>\n

    <\/a>3. The Conjugate Acid Is Always A Better Leaving Group<\/h2>\n

    To put some numbers on it, if you look at a pKa<\/sub> table, you’ll see that the pKa<\/sub> of H2<\/sub>O (water) is about 14 (See post: How to Use A pKa<\/sub> Table<\/a><\/em><\/span>). Its conjugate base is HO(\u2013). When we add acid, water becomes H3<\/sub>O(+), which has a pKa<\/sub> of \u20131.7. It’s a much stronger acid, in other words, and therefore its conjugate base (water, H2<\/sub>O) is much weaker<\/strong>. In other words, by adding acid, we’ve made it a better leaving group.<\/p>\n

    This is a general phenomenon, by the way – the conjugate acid will always be a better leaving group.\u00a0<\/strong><\/p>\n

    It applies not only to OH, but other functional groups as well. For example, ethers (R\u2013O\u2013R) are some of the most unreactive species you’ll meet. However, if you add a very strong acid to an ether, you can break it open to give an alcohol and an alkyl halide:<\/p>\n

    \"acid<\/p>\n

    <\/a>4. Halogens Can Be Made Into Better Leaving Groups By Adding Lewis Acids Such As Silver Ion (Ag+)<\/h2>\n

    We can take advantage of this to make\u00a0halogens<\/strong>\u00a0better leaving groups too. Halogens don’t react with H+ as readily as alcohols and ethers do, but they do<\/em> react with certain Lewis acids\u00a0[remember – a Lewis acid accepts a lone pair<\/em><\/span>].\u00a0One Lewis acid that gets a lot of attention with halides is Ag+ (silver ion).<\/p>\n

    You might see it as AgNO3<\/sub> or AgBF4<\/sub> \u00a0(the counter-ion NO3<\/sub>(-) or BF4<\/sub>(-) is just a spectator here<\/em><\/span>). When Ag+ combines with a halogen such as Cl, the resulting species R-Cl-Ag (+) has a considerably weaker C-Cl bond, meaning it can better participate in substitution reactions. With secondary or tertiary alkyl halides, the result is usually formation of a carbocation.<\/p>\n

    One neat trick is that silver halides are insoluble in water, so if water is chosen as solvent, loss of AgCl is irreversible<\/strong>. The carbocation is then trapped by the water solvent.<\/p>\n

    \"lewis<\/p>\n

    In the end, it’s still the same phenomenon here. Whether we use a Br\u00f8nsted acid or a Lewis acid, the conjugate acid is always a better leaving group<\/strong>, and acid can be helpful for getting substitution reactions to proceed at a much faster rate than they would otherwise.<\/p>\n

    In the next post, let’s compare the SN<\/sub>1 and SN<\/sub>2 reactions.<\/p>\n

    Next Post: Comparing The SN1 and SN2 Reactions<\/strong><\/a><\/p>\n

    \n

    <\/a>Notes<\/h2>\n

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