Physical properties of alkanesmostly `even Steven’

Boiling point vs #C in unbranched Boiling point vs #C in unbranched chain alkanes (Cchain alkanes (CnnH H 2 n+22 n+2))

Melting point vs #C in unbranched Melting point vs #C in unbranched chain alkanes (Cchain alkanes (CnnH H 2 n+22 n+2))

Alkane physical properties (continued)

Straight chain boils higher than equivalent branched chain Straight chain boils higher than equivalent branched chain with same #C (`packing’ matters)with same #C (`packing’ matters)

Example:n pentane 36 Example:n pentane 36 ooC > isopentane 28 C > isopentane 28 oo C > neopentane C > neopentane oo10 C10 C

36 o C

28 o C

10 oC

More More spherical spherical means less means less stickysticky

Lower bp, less stickyLower bp, less sticky

bp (bp (ooC)C)

3636

2828

1010

INTUITION:MORE SPHERICALLY SHAPED ISOMERS ARE LESS STICKY.

VISUAL INTUITION ON BP TREND

Alkane physical properties (continued

Density variations reflect local molecular effects until Density variations reflect local molecular effects until chains get past 3 Carbons .chains get past 3 Carbons .

A brief look at rotational isomers (pp 80-85)A brief look at rotational isomers (pp 80-85)

An isomeric set = isomers share the same formula: e.g. CAn isomeric set = isomers share the same formula: e.g. C55HH12 12

Structural isomersStructural isomers

Structurally distinctStructurally distinct

Physically/chemically differentPhysically/chemically different

Different namesDifferent names

bp bp ((ooC)C)

3636

2828

1010

Rotational isomersRotational isomers

Structurally similarStructurally similar

Physically/chemically similar Physically/chemically similar (but not =)(but not =)

Same nameSame name

bpbp

decreasingdecreasing

Sum of all its Sum of all its rotational rotational isomersisomers

(helps explain (helps explain bp range)bp range)

EXTENSION OF INTUITIVE MODEL OF BOILING POINT (BP) VS STRUCTURE FOR ROTATIONAL ISOMERIC FORMS

BPBP

decreasingdecreasingDOES BP TREND MAKE SENSE ?

yes

Ethane’s rotational conformers

`sawhorse’ 3D projections of ethane conformers

Newman projection equivalents (p. 81)

Potential energy vs. dihedral angle for ethane (see also: Fig. 3.7)

One `staggered’ version of butane

Other staggered versions of butane

Like the staggered forms, there are several kinds of eclipsed conformations

Eclipsed versions of butane

4 distinct kinds of butane conformers (pp. 82-3)

same

same

Potential energy vs. torsion angle for butane (see also: Fig 3.9 p. 84)