Lecture 2 - properties of Alkanes

Lecture 2 - properties of Alkanes - BME 201, August 31,...

Info iconThis preview shows pages 1–10. Sign up to view the full content.

View Full Document Right Arrow Icon
BME 201, August 31, 2010 Suggested problems review material: Vollhardt, 6 th Ed. Chapter 2, problems no. 40, 41, 45, 46; Chapter 3, problems no.17, 18, 22, 23, 27, 46, 48 Todays lecture is based on Vollhardt, pages 73-102, 104-118, 122-124, 126- 127, 131-150, 147-158, 169-180, 187-189 ff and material from handouts. 1) Structural and Physical Properties of Alkanes 2) Representative Reactions You will find it easier to do the suggested problems and pass future quizzes and exams if you go over the reading prior to attempting to solve the suggested problems. 1
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Structural and Physical Properties of Alkanes 2-6 Alkanes exhibit regular molecular structures and properties. Alkane structures are regular. The carbon atoms are tetrahedral (bond angles close to 109 o ), C-C bond lengths all ~1.54 Å, and C-H bond lengths all ~ 1.10 Å. The 3-D structures are depicted by the dashed/wedged link notation. 2
Background image of page 2
The physical constants of alkanes follow predictable trends : Why? Vollhardt page 77 3
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Types of Intermolecular Forces that may hold molecules together Type of Force Relative Strength Low Molecular Analog Polymers Dispersion or Van der Waals (intermolecular) Weak Methane Hexane Polyethylene Polypropylene Dipole-Dipole Medium CH 3 Cl CH 3 CO 2 CH 3 PVC PMMA Hydrogen bonding Strong H 2 O CH 3 CONH 2 Proteins Cellulose Polyamide Electrostatic or Ionic Very Strong NaCl CH 3 CO 2 - Na+ Ionomers from B. Daly slides, LSU 4
Background image of page 4
The physical forces between alkane molecules are due to London forces . These attractive forces arise from the correlation of electron motion on neighboring molecules. 5
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
With alkanes, London forces and therefore melting points increase with increasing molecular size (increased surface area contacts). Branched alkanes have smaller surface areas than their linear isomers and also cannot pack together as efficiently. Their melting points are usually lower than the corresponding linear isomers. Highly compact branched molecules (symmetrical) are exceptions. 6
Background image of page 6
Rotation about Single Bonds: Conformations 2-7 Rotation interconverts the conformations of ethane. The barrier to rotation of the two methyl groups in ethane is ca. 2.9 kcal/mol. Since this amount of energy is available at room temperature from molecular collisions, the methyl groups are said to have free rotation . During the rotation, the conformation moves from the staggered to the eclipsed , and to a second staggered conformation. 7
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Newman projections depict the conformations of ethane. The Newman projection is an alternative to using the dashed/wedged notation: 8
Background image of page 8
There can be more than one staggered and one eclipsed conformation: conformational analysis of butane. There are two different types of staggered arrangements in butane. One in which the two terminal methyl groups are 180 o apart ( Anti ), and two in which they are 30 o apart ( Gauche ).
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 10
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 02/20/2012 for the course BME 201 taught by Professor Schmidt during the Fall '11 term at Purdue University-West Lafayette.

Page1 / 61

Lecture 2 - properties of Alkanes - BME 201, August 31,...

This preview shows document pages 1 - 10. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online