Hydrogen bonding, generalized rules

A peroxide reverses the order of addition because it changes the mechanism of the reaction in a way that causes Br to be the electrophile. Markovnikov s rule is not followed because it applies only when the electrophile is a hydrogen. The general rule— that the electrophile adds to the sp carbon bonded to the greater number of... 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

Olefin Complexes. Silver ion forms complexes with olefins and many aromatic compounds. As a general rule, the stabihty of olefin complexes decreases as alkyl groups are substituted for the hydrogen bonded to the ethylene carbon atoms (19). 

Ethylbenzene and toluene are hydrogenated faster than benzene over Cu-ZnO, contrary to the general rule. Such behavior is evidence for a t bonded intermediate, the surface concentration of which increases with the increasing electron-donating ability of the system (5(5). 

The electronegativity of sodium and chlorine differ by 2.23, whereas the difference between hydrogen and oxygen is only 1.24 (see Table 7.1). As a general rule, molecules made up of two atoms with electronegativity differences greater than 2.0 form ionic bonds. Molecules whose atoms have electronegativity differences of less than 2.0 form covalent bonds. Ionic-bonded salt and covalent-bonded water conform to that rule. 

Trouton s rule states that for most normal liquids the entropy of vaporization per mole 21 e.u. By a normal liquid, we mean which is not associated. In general association in the liquid state may be expected when intermolecular forces of a dominant type operate. Dipole moments, hydrogen bonding etc., lead to this situation. Abnormally high boiling points are a consequence of molecular association in the liquid state. 

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