Halogen Derivatives - incomplete Flashcards
What are halogenoalkane & halogenoarenes
halogenoalkane: halogen bonded to sp3 hybridised C atom
halogenoarenes: halogen derivatives of arenes (X directly bonded to benzene ring)
how to name:
benzene-CH2Cl
(chloromethyl) benzene
Brackets for the chloro cuz it’s not bonded directly to the benzene ring
Primary/Sec/Tertiary halogenoalkanes
Primary - 1 R attached to the C atom bearing the halogen
Secondary - 2 R “
Tertiary - 3 R “
Physical properties of halogenoalkanes (3)
BP:
- BP of halogenoalkanes RX (pdpd & idid) > corresponding alkane (idid)
- For halogenoalkanes RX with the same X [/R], bp increases as the size of R [/X] increases. As the size of R [/X] increases, there is a larger and more polarisable e- cloud as the no. of e- per molecule increases.
Hence, strength of idid increases
Solubility:
- Poor solubility in water - altho they are non polar, they are not able to interact with water molecules via hydrogen bonding.
- Soluble in organic solvents - able to interact with these solvent molecules via idid forces
Density:
Density increases as X increaeses and atomic no. of X larger = denser
- fluoroalkanes & monochloroalkanes are less dense than water
- in general, chloroalkanes with 2 or more Cl atoms, bromoalkanes & iodoalkanes are denser than water
Halogenoalkanes can be prepared from (3)
Alkanes, alkenes and alcohols
Preparing halogenoalkanes from alkanes
Eqn, reagents & conditions, mechanism
Why is this not a gd method to prepare halenoalkanes?
R-H + X2 ————–> R-X + HX
R&C: X2 (limiting), uv light/sunlight/heat
M: Free-radical substituation
Not a very gd method as poly-substitution can occur, leading to a mixture of products. Using limiting amt of X2 helps to minimise poly-substitution
Preparing halogenoalkanes from alkenes
- halogenoalkane with only 1 halogen atom per molecule
- halogenoalkane with 2 adjacent halogen atoms per molecule
Eqn, reagents & conditions, mechanism
- halogenoalkane with only 1 halogen atom per molecule
C=C + HX (g) ————> C - C
| |
H X
R&C: dry gaseous HX, rm temp
Mechanism: electrophilic addition - halogenoalkane with 2 adjacent halogen atoms per molecule
C=C + X2 —————–> C - C
| |
X X
R&C: X2 in CCl4, rm temp
Mechanism: electrophilic addition - can form mixed halogen too - less electronegative one will be δ+
eg CH2=CH2 + IBr ——-> CH2ICH2Br
R&C: IBR in CCL4, rm temp
Mechanism: electrophilic addition
Preparing halogenoalkanes from alcohols (4)
Eqn, reagents & conditions, mechanism
General rule when to use which for which halogen?
- dry HX, heat
E.g. R-OH +HBr ——–> R-Br + H2O - PX3 *anhydrous condition must be used becuz PX3 reacts with water
E.g. 3R-OH + PBr3 ——–> 3R-Br + H3PO3
E.g. 3R-OH + PI3 — red P, I2 —-> 3R-I + H3PO3 - PCl5
E.g. R-OH + PCl5 ———-> R-Cl + POCl3 + HCL
*Test for alcohol -OH grp: white fumes of hydrogen chloride, which turns damp blue litmus paper red - SOCl2 *sometimes preferred cuz prod. are gases, easy to remove
E.g. R-OH + SOCl2 ————> R-Cl + SO2 + HCl
R-OH —> R-Cl | PCl5 or SOCl2, rm temp
R-OH —> R-Br | PBr3 or dry HBr, heat
R-OH —> R-I | red P, I3
Defn of nucleophile
Is a species that is able to donate an e- pair to an electron-deficient atom to form a covalent bond
Polar carbon-halogen bond
which has a partial positive charge?
the C-X bond in R-X is polar as the halogen atom is more electronegative than the carbon atom, hence the C atom has a partial +ve charge and is said to be e- deficient
How does nucleophilic substitution work
2 key differences of SN2 & SN1 mechanisms + general facts? abt it
SN2:
Kinetics: rate is directly prop to [OH-]
Stereochemistry: Enantiomerically pure reactant gives enantiomerically pure prod
Since the overall order of rctn is 2, it involves both halogenoalkane and nucleophile in the slow single step. Hence, we can conclude that the rctn is single-step and bimolecular.
SN1:
Kinetics: rate is independent of [OH-] (Order of rctn is 1)
Stereochemistry: Enantiomerically pure reactant gives racemic mixture of prod
Since the overall order of rctn is 1, it involves only halogenoalkane in the slow step. Hence, we can conclude that the rctn is more than 1 step and unimolecular.
Nucleophilic substitution - SN2 mechanism:
what S, N & 2 stand for
Description
Checklist for mechanism (5)
S stands for substitution
N stands for nucleophilic
2 stands for 2 rct particles involved in the rate determining step (bimolecular)
Nucleophilic attack:
- In the halogenoalkane molecule, the C-X bond is polar with the C atom acquiring a partial +ve charge since it is less electronegative than the halogen atom.
- During the rctn, the nucleophile attacks the partially positively charged C atom from the side DIRECTLY OPP the halogen atom (backside attack)
- In the transition state, there is simultaneous partial formation of C-Nu bond and partial cleavage of the C-X bond
Checklist:
- name of mechanism
- δ+ on C & δ- on X
- lone pair of e- on Nu-
- curly arrows to show flow of e-
- inverted prod (inversion of configuration)
Why must nucleophilic attack be from the backside?
Xδ+ blocks the approach of the nucleophile from the front side. It also repels the nucleophile since both are e- rich
Energy profile diagram of SN2 rctn
