Physical properties of alkanes mostly `even Steven’ Boiling point vs #C in unbranched chain alkanes (C n H 2 n+2 ) Melting point vs #C in unbranched chain alkanes (C n H 2 n+2 )

Alkane physical properties (continued) Straight chain boils higher than equivalent branched chain with same #C (`packing’ matters) Example:n pentane 36 o C > isopentane 28 o C > neopentane o 10 C More spherical means less sticky Lower bp, less sticky

bp ( o C) INTUITION: MORE SPHERICALLY SHAPED ISOMERS ARE LESS STICKY. VISUAL INTUITION ON BP TREND

Alkane physical properties (continued Density variations reflect local molecular effects until chains get past 3 Carbons.

A brief look at rotational isomers (pp 80-85) An isomeric set = isomers share the same formula: e.g. C 5 H 12 Structural isomers  Structurally distinct  Physically/chemically different  Different names bp ( o C) Rotational isomers Structurally similar Structurally similar Physically/chemically similar (but not =) Physically/chemically similar (but not =) Same name Same name bpdecreasing Sum of all its rotational isomers (helps explain bp range)

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

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