HALOGENOALKANE Flashcards
HALOGENOALKANE
alkanes with atleast one or more hydrogen replaced by a halogen.
polarity
halogens are more electronegative tham carbon so withdraws electron density from the carbon which leaves the carbon with a partial positive charge and the halogen with a partial negative charge. the partial positive charge doesnt has enough electrons so it can be attatcked by nucleophiles.
nucleophiles
a nucleophile is an electron pair donor. donates an electron pair to somewhere where there is not enough electrons.
example of nucleophiles reacting with halogens
ammonia, cyanide, hydroxide.
electronegativity
ability of an atom to attract the electron density in a covalent bond.
boiling point of halogenoalkane
increases down the group. strength of intermolecular force governs the boiling point.
more electrons - stronger vamderwaals force - more energy required - higher boiling point. also branching reduces boiling point, vanderwaals forces are weaker, less surface area.
solubility of halogenoalkane
thumb rule - like dissolves like. so polar subsatnces dissolve in polar solvent and same with non polar substanes.
halogenoalkanes are insoluble or partially soluble in water but highly soluble in hydrocarbons. as we increase the lenght of thecarbon chain, there are more carbon - carbon or carbon - hydrogen bonds. since these bonds are non polar, a large region of these molecules are also non polar. as the length increases, the solubility in water decreases.
explain why 1- fluoro propane has a higher melting point than fluoro ethane
fluoroethanes structural formula is CH3CH2F and 1- fluoro propane is CH3CH2CH2F. as the length of the carbon chain increases from fluoroethane to 1 - fluoro propane, the number of electron increases which means the molecule experiences more vander waals forces which increases the melting point.
nucleophilic substitution
in a nucleophilic substitution reaction, a nucleophile attacks a polar molecule., kicks out a functional group and settles itself down in the place.
mechanism of nucleophilic substitution reaction
the lone pair of electrons on the nucleophile attacks a slightly positive charge on the carbon which is shown by a black curly arrow. the klone pair of electrons create a new bond between nucleohile and carbon. the carbon can only be bonded to 4 other atoms so the addition of nucleophile breaks the bond between carbon and halogen which is shown by another curly arrow. the pair of electrons from the carbon hydrogen bond are taken away by halogen and becomes a lone pair.
reagent
substance that is physically added to the system. eg in NAOH AND KCN, oh and cn are the nucleophiles and K and NA is the reagent. however nh3 is both nucleophile and reagent.
nucleophilic substitution ( reaction with hydroxide)
when halogenoalkane undergo nucleophilic substitution with hydroxide ion, alcohol is formed always.
naoh and koh are some example of reagents which always needs to be aqueous for the reaction.
why do halogens take part in nucleophilic substitution
in general, halogens are more electronegative than carbon. this means that halogens contain a polar bond. so when they react with nucleophiles, they can act as electrophiles.
Reaction with cyanide ions
halogenoalkanes react with cyanides to form nitriles.
warm ethanolic potassium cyanide
reflux are needed for this rxn to occur.
Reaction with ammonia
warm a halogem with excess ethanolic ammonia, in a sealed tube, the ammonia swaps places within the halogen to form an amine. HEATING, EXCESS AMMONIA AND PRESSURE IS NEEDED.
