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HALOGEN DERIVATIVES
replacement of H-atoms in aliphatic/aromatic…
HALOGEN DERIVATIVES
- replacement of H-atoms in aliphatic/aromatic hydrocarbons by halogen atoms
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Nomenclature
- alkyl halide=alkyl grp+halide. eg methyl iodide
according to iupac:-
- alky halides-haloalkanes
- aryl halides-haloarenes
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PHYSICAL PROPERTIES
- they are different in alkyl halides than corresponding alkanes
- eg boiling pt of alkyl halides determined by polarity of c-x bond and size of halogen atom
Nature of intermolecular force
- halogens are more electronegative than c
- so c atoms that carry x develop partial +ve charge and x has partial -ve charge
- so c-x bond in alkyl halides is polar covalent bond
- size of x increases from F to I so c-x bond length increases
- so c-x bond strength decrease with increase in size of x
Boiling point
- alkyl halide>alkanes
due to higher polarity and higher molecular mass
- within alkyl halide,for given alkyl grp boiling pt increases with increase in atomic mass of halogen RI>RBr>RCl>RF
- boiling pt increase with increase in no of c
- for isomeric alkyl halide,boiling pt decreases in increase in no of branching
Solubility
- insoluble in water
- due to inability of alkyl halides to form hydrogen bonds with water
- attraction betn alkyl halide molecule is stronger than attraction betn alkyl halide and water
- alkyl halide are soluble in non-polar organic solvents
- aryl halides are also insoluble in water but soluble in organic solvents
CHEMICAL PROPERTIES
Nucleophilic substitution reactions for haloalkanes
- when a grp bonded to a carbon in substrate is replaced by another grp to get a product with no change in state of hybridisation of that carbon-subsitution reaction
- in c-x bond carbon is +vely polarised so c-x bond is an electrophilic centre
- hence it has tendency to react with nucleophiles to give nucleophilic substitution reactions(SN)
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Mechanism of SN reactn
- halogen atom gets detached from the carbon and a new bond is formed betn electrophillic carbon and nucleophile
- covalently bonded halogen is convertd into halide ion so 2e- constitutiong the original covalent r carried away by halogen
- so the halogen atom is the leaving grp(grp which leaves the carbon by taking away bond pair of e-)
- substrate undergo 2 changes;-c-x undergo heterolysis and new bond is formed betn carbon and nucleophile using 2e- from nucleophile
- the sequence of these changes and the way they take place in sn reaction=mechanism of Sn reactn
SN2 MECHANISM
- CH3Br+OH- gives CH3OH+Br-
rate=k(CH3Br)(OH-)
- The reactn follows 2nd order kinetics(rate of this reactn depends on conc of 2 reacting species)
- the above reactn is single step and slow step
- single step mechanism with simultanous bond breaking and forming
- backside attack of nucleophile to avoid steric repulsion and electrostatic repulsion between incoming nucleophile and the leaving grp
- in transition state(T.S) the nucleophile and leaving grp r bonded to carbn with partial bond and partial -ve charge(hence total -ve charge is diffused)
- (T.S) contains pentacoordinate carbon having 3 sigma bonds in one plane (bond angle 120) and 2 partial covalent bonds along a line perpendicular to this plane
- in SN2 in chiral carbon(in optically active substrate) product has opp configuration than substrate.this inversion of configuration is called walden inversion.it s due to backside attack of nucleophile
SN1 MECHANISM
- tert butyl bromide+OH- gives tert butyl alcohol
rate=k[(ch3)3cbr]
- follows 1st order kinetics (rate of reaction depends on conc of only 1 species i.e substrate (tert butyl bromide)
- it is seen that nucleophile doesnt influence the reactn so tert butyl bromide reacts with OH- in 2 step mechanism
- in slow step(1st step),c-x bond in substrate undergoes heterolysis and further in fast step nucleophile uses its e- pair to form new bond with carbon undergoing change
- 2 step mechanism
- heterolysis of c-x bond in slow and reversible 1st step to form planar carbocation intermediate
- attack of nucleophile on carbocation intermediate in fast 2nd step to form product
- in SN1 in chiral carbon( in optically active substrate) product is nearly racemic.so SN1 reactn mainly proceeds with racemization hence both enantiomers of product r almost equal.racemization in SN1 is result of formation of planar carbocation intermediate.nucleophile can attack planar cabocation frm either side which result in formation of both enantiomers of product
FACTORS INFLUENCING
nature of substrate:
- SN2; T.S is crowded(pentacoordinate) so SN2 is favoured in primary halides and least favoured in tertiary halides
- SN1;favoured in tertiary halides and least favoured in primary halides as its intermediate doesnt have crowding so bulky alkyl grps can be accomodated
- also favour tertiary because intermediate is stabilised by +I effect of alkyl substituents and by hyperconjugation effect of alkyl substituent containing alpha hydrogen
- tertiary halides undergo nucleophilic substitution by SN1 mechanism and primary by SN2.
- secondary react by either of the mechanism or by mixed mechanism depending on exact condition
nucleophilicity of the reagent
- more powerful nucleophile attacks substrate faster and favours SN2 mechanism
- rate of SN1 mechanism is independent of nature of nucleophile
- nucleophile does not react in slow step of SN1,it waits till intermediate(carbocation)is formed and reacts fast with it
solvent polarity
- in SN1 good ionizing solvents,polar solvents stablize ion(carbocation) by solvation
- solvation of carbocation is poor
- anions are solvated by H-bonding solvents-protic solvents,hence SN1rectn proceeds more rapidlyin polar protic solvent
- polar protic solvents decrease rate of SN2 reactn
- aprotic or solvents of low polarity will favour SN2 reactn
Elimination Reaction:Dehydrohalogenation
- alkyl halide with atleast one beta -H is boiled with alcoholic solution of KOH,it undergoes elimination of H-atom from beta-C and X-atom from alpha-C resulting in formation of alkene
- also called beta-elimination
- since X and H is removed in this reaction hence known as dehydrohalogenation reaction
- if there are 2 or more non-equivalent beta-H atoms in halide
SAYTZEFF ELIMINATION
- since diff products of elimination do not form in equal proportion,hence he formulated an empirical rule
- in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms,and alos they r more stable
- stability of alkyl substituted alkenes
CH2 = CHR < RCH = CHR < R2C = CHR < R2 C = CR.
Monohalogen compounds
alkyl halides/haloalkanes
- halogen atom is bonded to sp3 hybridized carbon - (part of saturated c-skeleton)
- can be primary secondary or tertiay depending on which carbon it is attached
allylic halides
- halogen is atom is bonded to a sp3 c-atom next to a c-c double bond
benzylic halide
- halogen atom is bonded to a sp3 carbon which is further bonded to an aromatic ring
vinylic halide
- halogen atom bonded to sp2 c-atom of aliphatic chain
- is a haloalkene
haloalkyne
- halogen atom is bonded to a sp hybridised carbon atom
aryl hallide /haloarene
- halogen atom is directly bonded to sp2 c-atom of aromatic ring
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Optical isomerism in halogen derivatives
- carbon atoms in a molecule which carries 4 diff grps/atoms is called a chiral carbon atom
- chiral molecule cannot superimpose perfectly on its mirror image so chiral molecule and its mirror image are not identical
- chiral molecule and its mirror image have same structural and molecular formula but spatial arrangement of 4 diff grps around chiral atom is diff
i.e they are stereoisomers of each oher
- relation betn chiral molecule and its mirror image is chirality
optical isomerism
- stereoisomerism in which the isomers have diff spatial arrangements of groups/atoms around a chiral atom
- optical isomers differ from each other by measurable property-optical activity
- isomerism in which isomeric compounds have diff optical activity
optical activity
- when an organic compounds like sugar soln,lactic acid is placed in the path of plane polarized light,transmited light has oscillation in diff plane than original
- so the incident light undergoes rotation of its plane of polarization
- compounds which rotate the plane of plane polarized light=optically active compounds
- expressed numerically in terms of optical rotation
Dextrorotatory
- compound which rotates the plane of plane polarized light towards right
- designated by d- or (+)
Laevorotatory
- compound which rotates plane of plane polarized light towards left
- designated by l- or (-)
Enantiomers
- optical isomers which are non-superimposable mirror image of each other
- have equal and opposite optical rotation
- eg. 2-chlorobutane exist as a pair of enantiomers
- identical physical prop(mp,bp,density,refractive index) except sign of optical rotation
- magnitude of OR is equal
- identical chem prop except toward optically active reagent
- equimolar mixture of enantiomers (dextroratatory and laevorotatory)=racemic modification or racemic mixture
Racemic modification
- optically inactive as molecules of one enantiomer is cancelled by equal and opposite OR due to molecules of other enantiomer
- designated by (dl) or plus minus sign
