Chapter 13 - Halogenoalkanes
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what is a nucleophile
an electron pair donor
why are nucleophiles attracted to the carbon atom in a carbon halogen bond
the carbon in the C-X bond is delta positive because of the large electronegativity difference between the 2 atoms the nucleophile is attracted to the slight positive charge
state and explain the trend in bond strength between a carbon and a halogen going down the group
the bond strength decreases because the size of the halogen increases so the shared pair or electrons is farther from the nucleus so the attraction is decreases which decreasing the bond enthalpy
what is the significance of the decreasing trend in bond enthalpies down the group in the carbon halogen bond
iodoalkanes (with the weakest carbon-halogen bonds) react the fastest, while fluoroalkanes (with the strongest bonds) react the slowest.
outline the steps in a nucleophilic substitution reaction
A nucleophile approaches the halogenoalkane, which has a partially positive carbon atom (δ+). The nucleophile donates its lone pair of electrons to the δ+ carbon, forming a new covalent bond. The original bond between the δ+ carbon and the halogen breaks heterolytically as the halogen atom takes both the shared electrons. The halogen departs as a halide ion (X-), being replaced by the nucleophile.
why are nucleophilic substitutions with cyanide important
the length of the carbon chain increases by one
what conditions are needed for elimination mechanisms to take place
ethanol used as solvent high concentration high temp no water present
outline the mechanism of elimination reaction
The OH- ion uses its lone pair to form a bond with one of the hydrogen atoms on the carbon next to the C-Br bond. These hydrogen atoms are very slightly B+. The electron pair from the C-H bond now becomes part of a carbon-carbon double bond. The bromine takes the pair of electrons in the C- Br bond and leaves as a bromide ion (the leaving group).
give the conditions of substitution and elimination reactions for haloalkenes with hydroxide ions
Hydroxide ions at room temperature, dissolved in water (aqueous), favor substitution. Hydroxide ions at high temperature, dissolved in ethanol, favor elimination.
name the properties of CFCs
non toxic, non flammable, non reactive, volatile, stable
why are CFCs harmful to the environment
in the upper atmosphere they decompose and release chlorine free radicals which act as a catalyst in the decomposition of ozone into oxygen. ozone acts as a shield from UV rays, so less ozone means more global warming as more UV can get through and reflect as infrared
what is the role of hydroxide ions in elimination reactions
a base
what were CFCs used as
propellants, refrigerants, solvents
give alternatives to CLCs name their properties
HFCs - These do not contain chlorine, so they do not harm the ozone layer, but they are powerful greenhouse gases. Hydrocarbons - These substances decompose rapidly but are flammable and contribute to greenhouse gas emissions.
FACT CHECK THIS WHEN YOU CAN! what must cyanide by dissolved in to produce amines
ethanol