What is an unbalanced alkene

Addition of hydrogen halides to asymmetrical alkenes - lesson from November 22, 2016 in the advanced course chemistry 11

The Electrophilic addition (AE) is a reaction mechanism in organic chemistry in which unsaturated hydrocarbons (alkenes or alkynes) react with different classes of substances. A common feature of electrophilic additions is that the reaction is initiated by the attack of an electron-loving particle, the electrophile, on the double or triple bond.

Example: addition of halogens to alkenes

Halogen molecules can be added to the double bond of alkenes in a two-step mechanism. The halogen molecule first interacts with the double bond of the alkene, which polarizes the halogen molecule and finally splits it heterolytically into a bromide ion and a bromonium ion (Br +).

The first step is the electrophilic addition of the bromonium ion (Br +). Then the bromide ion nucleophilically attacks the positivized carbon atom, whereby the saturated dibrominated product is obtained.


Class transcript from November 22nd, 2016 - advanced course chemistry 11

The Markovnikov Rule (after Vladimir Vasilyevich Markovnikov) states that when hydrogen halides are added to asymmetric alkenes, the hydrogen atom is always bonded to the carbon atom that is already richer in hydrogen. The halogen atom is accordingly bound to the lower hydrogen, i.e. more highly substituted, carbon atom. The same effect can be observed with the addition of water: the H atom is bonded to the lower substituted carbon atom, the OH group to the higher substituted carbon atom. The motto is: "He who has will be given".

Source: http://de.wikipedia.org/wiki/Markownikow-Regel

Excursus: For a more detailed explanation of the Markovnikov rule:

Inductive Effects

Movie tips:

The inductive effect


Understanding inductive effects / positive and negative inductive effect


TheInductive effect orI effect is a charge-changing effect in organic chemistry, which is both+ I effect("Electron-pushing") as well as−I effect ("Electron-withdrawing") occurs.

There are two types of I-effects: the+ I effect (read: positive inductive effect) and the−I effect (read: negative inductive effect).

A more electronegative atom has an −I effect, so that the electron density in the other atom is reduced.

With a + I effect, the electrons are pushed away from one atom and thus the electron density on the other atom increases.

In order to compare the strength of the inductive effect of atoms or groups of atoms, the electronegativity of the substituent is compared with the electronegativity of hydrogen.

The different reaction rates of different alkenes in the electrophilic addition can be attributed to inductive effects. Electrophilic addition is facilitated by the electron-donating + I effect of alkyl groups, while electronegative halogen atoms accelerate electrophilic addition.

Source: http://www.jagemann-net.de/chemie/chemie12lk/kohlenwasserstoffe/kw12_lk.php#Markownikow

Justification of the Markownikoff rule: An old class transcript from 2008 (Lk Chemistry 11)