RX Flashcards
Why does R-X have a higher BP/MP than its alkanes
RX requires↑ amt of energy to overcome the stronger pdpd interactions within polar R-X molecules compared to the weaker idid interactions within non-polar alkane molecules.
Why does BP of RX increase down the group/Why C-I>C-Cl>C-Br
Size of e- cloud↑, resulting in a ↑ polarisable e- cloud, resulting in stronger idid between RX that requires↑ energy to overcome, thus BP↑ down group
Why is RX insoluble in water
RX cannot form H-bonds w/ water, thus energy released when pdpd formed b/w RX and H20 is insufficient to overcome pdpd b/w RX mol & strong H-bonds formed b/w H2O molecules
Why is RX soluble in non-polar solvents
Energy rel from idid b/w np molecule and RX is sufficient to overcome the similar idid int in np mol and pdpd b/w RX
how to synthesise RX from alkanes, R&C
FRS, limited X2, uv light
how to synthesise RX from alkenes, R&C (2 methods)
1) HX(g). rtp
2) X2 in CCl4, rtp, dark(for dihalogenoalkane)
how to synthesise RX from alcohols, R&C, and products
1) PCl5, rtp -> RX, POCl3, HCl(g)
2) SOCl2 in pyridine, heat -> RX + SO2(g) + HCl(g)
distinguishing test for RX(2)
1) warm w/ aq NaOH, dilute HNO3, aq AgNO3, AgX ppt formed(AgI yellow, AgCl white, AgBr cream)
Why is XS NH3 required for reaction of RX w NH3
To prevent multi-substitution of RX as primary amine formed from sub of NH3 has a LP and can act as a nucleophile and attack RX again
When RX undergoes Nu sub w NH3, why must the reaction be heated in a sealed tube
To prevent loss of Nh3 since NH3 has to be added in XS
Acidic Hydrolysis of Nitriles(R&c + products)
RCN + H+ + 2H2O -> RCOOH + NH4+
R&C: HCl(aq), HUR
RX and (CN) Nu sub
Ethanolic KCN, HUR
RX n NH3
XS conc, NH 3 in ethanol, heat in sealed tube
RX n alcohol(Nu sub)
NaOH(aq), HUR
Reduction of RCN(R&C + products)
RCN + 2H2-> RCH2NH2
H2(g), Ni cat, heat OR
LiAl4 in dry ether, rtp
Basic Hydrolysis of RCN(R&C + products)
RCN + OH- + H2O -> RCOONa + NH
NaOH(aq), HUR
Elimination of RX -> alkene
Ethanolic NaOH. HUR
adjacent carbon must have at least 1 H attached to it
Explain the inversion of configuration in the SN2 reaction
The OH- Nu attacks the E+ carbon atom from the opposite side of the leaving halogen atom, which causes the 3 bonds attached to the chiral carbon centre to turn inside out
SN2
1 step, 2 reactant,primary RX
SN1
2 step, 1 reactant, tertiary RX, benzene
Why does SN2 favour primary RX
The e- def carbon atom of primary RX allows easy approach of the Oh- nucleophile as there is less steric hindrance while tertiary carbon atoms will sterically hinder the approach of nucleophile
Explain why SN1 favours tertiary RX and benzenes
Tertiary RX gives a stable carbocation int as e- donating alkyl groups disperse the charge on C+ intermediate to a greater extent
C+ intermediate is sp2 hybridised, thus the benzene ring allows the positive charge to be dispersed into the pi e- cloud of BZ ring, thus stabilizing C+ int.
Explain why the resultant mixture of SN1 reaction does not rotate plane polarised light
Since C+ is trigonal planar, the OH- nucleophile can attack C+ from the top and bottom of the plane with equal probability. If rxtant is chiral, a racemic mixture that is optically inactive is formed as both mirror image enantiomers are formed in equal quantities therefore it does not rotate PPL
Relative reactivity of RX(F, Cl, Br, I)
RI is the most reactive while RF is the least reactive since RI has the longest bond and therefore has the weakest bond strength and therefore easiest to break the C-I bond
Explain why primary RX bonded to Benzene does not undergo SN2
Although RX is primary, it is bonded to benzene which is bulky in nature, resulting in high steric hindrance
Why are halogenoarenes inert to Nu sub
Cl p orbitals overlap side on with the pi e-cloud in benzene which gives C-Cl a partial double bond character
