The Mechanisms of Addition Reactions
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what are curly arrows used for
representing the movement of a pair of electrons
what does the pi bond around the C=C bond in alkenes have
a high electron density
what does this pi bond make alkenes attracted to
• species that are electron deficient - including molecules with polar bonds
why is hydrogen bromide a polar molecule
• because bromine is more electronegative than hydrogen • which can be shown with their partial charges • H(o+)-Br(o-) • o = sigma bond
what happens when a hydrogen bromide molecule approaches an ethene molecule
the slightly positive end of the HBr molecule is attracted to the electrons in the pi bond in the C=C
what is the HBr molecule called when it does this
an electrophile
what is an electrophile
a species that is electron deficient and therefore attracted to a negative charge
what are the two things curly arrows do with this type of reaction
• start from a bond and move to an atom - or start from a lone pair of electrons and move to an atom
what is the full name for an addition reaction of this type, between ethene and hydrogen bromide
electrophilic addition
what is electrophilic addition
an addition reaction that involves an attack by an electrophile
in a reaction between ethene and hydrogen bromide, which atom out of hydrogen and bromine would be attracted to the double bond and why
• the hydrogen would - because the bromine is more electronegative than hydrogen
how are the shared electrons between hydrogen and bromine now distributed now that their bond is broken
• both of the electrons go on the bromine atom • causing the hydrogen to to be partially positively charged • and the bromine to be partially negatively charged
why does it make sense for the electrons to be pushed towards the atom in the downwards direction rather than staying with the atom closer to the double bond
• the electron in the pi bond and the electrons in the covelant bond repel • causing the electrons to be pushed downwards and onto the bromine atom in this case
what is kind of bond breaking called
heterolytic fission
what happens in the first step in the reaction between ethene and hydrogen bromide
• the hydrogen atom in the hydrogen bromide reacts with the C=C bond breaking it • forming positively charged molecule and a negatively charged bromine ion
what is the positively charged molecule called and why
• a carbocation - because it has its charge on a carbon atom
what arrows would be used in this step
• the first arrow would point from the double bond to the hydrogen atom with o+ • the second arrow would point from the bond between the hydrogen and bromine to the bromine atom with o-
what is the second step in this reaction
• the two oppositely charged ions attract each other • and react to form a new covelant bond • forming bromoethane
what arrow would be used in this step
• the arrow from the bromine ion (specifically from the line pair of drawn electrons) • to the carbon atom with the positive charge
what is the difference between this kind of electrophilic addition and reactions that form halogen halides
the attacking bromine molecule in halogen halide reactions doesnt have a polar bond
what would happen to a bromine molecule if it were to approach the C=C bond anyway and why
• the molecule would become polar at that point • that is because as the bromine molecule approaches the bond, the electrons in the pi bond repel the electrons in the Br-Br bond • this causes one of them to be partially positive and the other to be partially negative • leading to an induced polar molecule
is the mechanism for the reaction between ethene and bromine the same and what is its name
• it is the same • the arrows and the order is the same, just that you have two bromines instead of 1 • the product is 1,2-dibromoethane
what is the effect of the attacking molecule and the alkene being unsymmetrical if they react
• there can be two possible products - because the atoms in the attacking molecule can be added into two different places
what is an unsymmetrical alkene
one in which the atoms on either side of the C=C bond are not the same