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Carbocations: Factors affecting their Stability

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A carbocation is a species where a carbon atom bonds to three carbon atoms and has a positive charge. Carbocations are electron deficient species and therefore very reactive and unstable . Anything which donates electron density to the electron-deficient center will help to stabilize them. Factors that stabilize them are the following:  Neighboring carbon atoms (inductive effect) Neighboring carbon-carbon multiple bonds (resonance effect) Neighboring atoms with lone pairs (resonance effect) How carbocations are stabilized by neighboring carbons atoms? The stability of carbocations decreases as the number of carbons attached to the C+ decreases. That means that tertiary carbocations are more stable than secondary that in turn are more stable than primary (Fig. 1). Fig. 1: Carbocation stability increases as methyl substitution increases around the electron deficient carbon C+. The methyl groups (-CH 3 ) are electron donating and therefore stabilize the positive charge (inductive ...
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Troubleshooting HPLC / Liquid Chromatography Systems – Peak Tailing

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In a previous post entitled  " TroubleshootingLC / HPLC Systems – Tailing Peaks " peak tailing due to bad columns and sample overload was explained and methods were proposed to fix it.  Peaks with strange shapes represent one of the most common problems that can arise in the LC/HPLC laboratory.  It should be first explained why tailing is bad : Peaks with tailing can be hard to integrate The precision and the reliability of assay methods involving distorted – peaks with tailing – is often poor when compared to good chromatography Resolution is low when peaks show tailing  Another cause of peak tailing is when wrong solvent is used to dissolve the sample. Wrong Solvent for Sample Peak tailing is observed when the volume and the kind of solvent used to dissolve the sample is not appropriate. Ideally a small volume of sample (dissolved in the mobile phase) should be injected. However, when a relatively large volume of a solvent stronger than the mobile phase is injec...
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The SN2 reaction: Substitution Nucleophilic Bimolecular

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Several distinct mechanisms are possible for nucleophilic substitution reactions at saturated C atoms, depending on the substrate, nucleophile, leaving group and the reaction conditions. By far the most common are the S N 1 and S N 2 mechanisms. Simple alkyl groups, methyl and primary alkyl groups always react by the S N 2 mechanism and never by S N 1 . Why methyl and primary alkyl groups react by the S N 2 mechanism? What are the steps in an S N 2 mechanism? Partly because the corresponding cations are unstable and partly because it is easy for the nucleophile to attack the C atom since it is surrounded by H atoms – and not by bulky alkyl groups. Experimental results show the following in S N 2 reactions: In this mechanism (Fig. 1)  there is backside attack 1 . The nucleophile attacks the carbon atom on the opposite side from the leaving group – 180 away from the leaving group – and the carbon atom turns inside out as the reaction proceeds The reaction is a one-step proc...
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