Skip to main content

Alkanes and radicals 4: Additionreactions of radicals

Alkanes and radicals

Addition of radicals on an alkene
Look at the two reactions below, the first reaction is in normal circumstances, which will result in a normal Markovnikov reaction (with the addition of a protic acid HX to an alkene, the acid hydrogen (H) becomes attached to the carbon with fewer alkyl substituents, and the halide (X) group becomes attached to the carbon with more alkyl substituents). In the second reaction, peroxide is used as condition, this will result in an anti-Markovnikov reaction. The peroxide will generate Br radicals in small amounts (this is the initiation for this anti-reaction). 

Let's see the peroxide mechanism in detail. As stated above, an alkyl peroxide is a radical initiator. The electrophile will add on the sp2 carbon which carries the most hydrogens (where also the radical with most alkyl groups will be formed). The termination reactions will have several other outcomes. In the major outcome, the radical will not be rearranged (which oftens occurs however with carbocations = 'carbocation shift'). 

Although, the radical addition on an alkene in combination with the peroxide effect can only happen with HBr, why? Because both propagation steps from the HBr process are exotherm, in contrary to HCl & HI. With these compounds, the normal electrophile addition occurs via the carbocation intermediate. 


Comments

Post a Comment

Popular posts from this blog

Alkylhalides: Introduction

Alkylhalides: an initiation Background information Alkylhalides are compounds (in this case salts ) that consist of an alkyl group and an halogen (seventh row in the table of Mendeljev).  Alkylhalides have good leaving groups , these groups are the atoms (or the atom) that will be substituted or eliminated during elimination or substitution reactions on the molecule. This fenomenon is caused by the polar bond formed between the halogen and the alkyl group, making the halogen more afferent to be attacked (substituated / eliminated) by a nucleophile. Nucleophile substitution reaction ( SN2 reaction) A nucleophile 'attacks' the alkylhalide, the leaving group - in this case the halogen - will be taken of the compound and will be replaced (SUBSTITUTED) by the nucleophile (hense the reaction its name). This reaction however is characterized and depended by several factors including: the solvent in which the reaction is taking place the reactivity or acti...
Post a Comment
Read more

Coca-Cola: Life - myth or truth?

"Coca-Cola Life" Recently the Coca-Cola company released a new beverage: the Coca-Cola Life drink. It should contain less sugar than the normal Coca-Cola drink; a part of the sugar is replaced with the recently approved sweetener from the Stevia plant (stevioglycosides). It's being sold as a 'natural' drink, but those stevioglycosides are as natural as the chemical sweetener aspartame found in Light and Zero drinks. However it is true that the amount of sugar is much lower (being reported as 33%, some say it's around 20%); yet approximately still 3-4 sugar cubes / 33 cl can (5-6 in a normal can). I bought it myself and found that the flavor has not really changed from the original Coca-Cola, however it does taste more "flat" - but definitely true it yourself. The outside of the can has the colour green. I personally do not associate this "healthy - natural" colour with Coca-Cola, but it's growing on me. In sum...
1 comment
Read more

The proteasome [1]: a crucial structure of protein degradation

The proteasome Proteins are constantly being synthesized and at the same time being degraded in each cell of our body. One of the most known mechanisms of proteolysis (= protein degradation) is the degradation done by lysosomes. However, an other important mechanism is degradation by the ubiquitin proteasome system (UPS). First of all, it is needed to understand that a cell maintains its protein concentration by a constant turn-over of proteins: when there is a lot of synthesis of a certain protein, this certain protein will also be degraded by a higher level, and vice versa. A protein that has to be degraded will be marked by a polyubiquitin chain which consists of 4 or more ubiquitinmolecules. This protein will be transported to a 26S proteasome. This structure in the cell is built by a 19S part and a 20S part. The 19S part mainly serves as a recognition and binding structure for the polyubiquitinated protein, and the 20S part will destroy the protein. Once attached to the...
Post a Comment
Read more