Simply Mechanisms 3a. Electrophilic Addition 3. Propene & HBr - Markovnikov's Rule HD

Find an accompanying mindmap here: http://franklychemistry.co.uk/simply_mechanisms/3_Simply_Mechanisms3_Propene_HBr_Markovnikovs_Rule.pdf One in a series of videos looking at organic chemistry mechanisms. This one looks at the electrophilic addition mechanism for propene reacting with hydrogen bromide. It explains the mechanism in detail, with the help of simple animations and molecular models. The H-Br molecule is polar. The C=C double bond is made up of a sigma and pi bond. All atoms attached to the C=C double bond in propene lie on the same flat plane. The pair of electrons of the pi bond spend most of their time either above or below this plane. They are more exposed to incoming electrophiles and the pi bond is weaker than the sigma bond. The propene molecule is an asymmetric alkene and this means there are two possible products: 1 bromopropane and 2-bromopropane. This is a 2-step mechanism. In the first step -slow step- rate-determining step - the pi bond breaks and the H-Br bond breaks. The H-Br bond breaks by way of heterolytic fission, meaning that both electrons of the bond go to the bromine atom. This creates a Br- ion and a H+ electrophile. The pair of electrons of the pi bond are then used to form a bond with the H+ electrophile. The other carbon of the C=C bond will have lost the electron it contributed to form the original pi bond. It will then have a positive charge, explaining why the intermediate, formed from the first step, is called a carbocation. AS propene is an asymmetric alkene it is possible to form either a primary carbocation or a secondary carbocation. Alkyl groups are ‘electron-donating’, helping to offset the electron-deficiency of the carbocations. The secondary carbocation is less unstable than the primary carbocation as it has 2 alkyl groups attached to the C+, whereas the primary carbocation has just 1. In the fast step the bromide ion and carbocation then combine to form the product. 2-bromopropane is the major product as it forms via the secondary carbocation intermediate. 1-bromopropane is the minor product as it forms via the primary carbocation. A reaction profile diagram helps to illustrate why the reaction is more likey to proceed via the secondary carbocation. Markovnikov’s Rule: That which has gets, tells us that the carbon of the C=C bond with the most H atoms directly attached to it is the most likely to get the H of the HBr.