a) mono-substitutions: bp trends

ALKANES ALKYL HALIDES

#C R-H R-F R-Cl R-Br R-I1 -162 -78 -24 + 3 +42 2 - 89 -32 +12 +38 +72 3 - 42 -3 +47 +71 +103 6 +69 +92 +134 +155 +180

trends verbalized:higher Carbon count and bigger halogens (X) mean higher bp in RX.

Physical Trends in Alkyl Halides (BP): a modest proposal (see also-text pp 289-90) …an example of physical organic chemistry

Dipole slightly decreasing---------

b) multiple halogen substitutions : bp trendsX CH3X CH2X2 CHX3 CX4

F -78 -51 -82 -128Cl -24 +40 +61 +77Br +3.5 +97 +149 +189I +42.5 +182 +218 +218

CH3CH2 X CH3CHX2 CH3CX3 CX3CX3

F -32 -25 -47 -78 Cl +12.3 +57 74 146 Br +38.5 +109 +153 318

trends verbalized: in the main, more halogens correlates with higher bp, but something else unconnected to dipole overrides

dipole effect and it’s not just a mass effect-see “anomalies”

Dipole increases high dipole 0 dipole

high dipole 0 dipole

RX CH3 F CH3 Cl CH3Br CH3 IMolecular mass (g/mol) 34 50 95 142

dipole moment(debyes) 1.86 1.89 1.82 1.62

boiling point (o C) -78 -24 +3 +42

c) “anomalies”

trends verbalized Neither permanent dipoles nor molecular mass are the whole story in allowing prediction of bp 1) why are equivalent masses of alkanes vs alkyl halides so different??? 2) how come a low dipole alkane beats a high dipole alkyl halide in bp ?…Speculation is that something connected to electronic cloud’s `puffiness’ (“P”) is involved

bp of ~equivalent R-H mass -32 +5 69 170

#C in alkane 2 3 6 10

dipole moment (debyes) 0 0.08 0.08 0.07

P=the ease of distortion of the electron cloud of a molecular entity by an electric field (such as that due to the proximity of a charged reagent). P is experimentally measured as the ratio of induced dipole moment (induced ) to the field E which induces it:

P = induced /E

Electronic puffiness =Polarizability, P, defined:

Test (-) charge (`E’)

Extreme polarizability(larger atoms)

Slight polarizability(smaller atoms)

Original, undeformed electron cloud border

Polarizability pictured:Position of original, undeformed cloud

Test distance

Polarizability quantified (sort of…)

Quantifying the intuitionP ~ 1

relevant electron density

The Doc’s little `intuition’less electrons per unit volume=> easier to push e- around=higher P

=>P inverse to electron density

relevant electron density ~ # valence e- in molecule molecular volume

P ~ 1 ~ 1 relevant electron density # valence e-

r3

~ # valence e- in molecule r3

=r3

# valence e-

CH3 F CH3 Cl CH3Br CH3 IMolecular mass (g/mol) 34 50 95 142

dipole moment(debyes) 1.86 1.89 1.82 1.62

boiling point (o C) -78 -24 +3 +42

# valence e- 14 14 14 14r(C-X) (10-10 m) 1.35 1.77 1.94 2.14

Testing the model

P ~ r3

#valence e-

P~ r3/#valence e- = 0.18 0.40 0.53 0.70

y = 229.72x - 118.2

R2 = 0.999

-100

-80

-60

-40

-20

0

20

40

60

0 0.2 0.4 0.6 0.8

P ~ 1/valence electron density

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d b

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pt,

C

F

Cl

Br

I

Plot of observed CH3X boiling point versus rough polarizability measure: P ~ 1/valence electron density

Test of the model: do you get linear relationship ?

Least square r2 ~ 1

Linear correlation of a model with physical observations makes for a….

Happy Physical Organic Chemistry Cat