Polar covalent bonds. Dipole Moments (µ) 1/26/2015. Molecular Dipole Moments (µ)

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1 Polar covalent bonds Dipole Moments (µ) A substance is said to be polar if there is an unsymmetrical distribution of electrons (i.e if its centres of positive and negative charges do not coincide). + + It is equal to the magnitude of the charge, e, multiplied by the distance, d, between the centers of charge. µ = e x d. It is expressed in Debye units (D). Not polar Polar 1 2 Molecular Dipole Moments (µ) Molecule must have polar bonds: - necessary, but not sufficient. Need to know molecular shape - because individual bond dipoles can cancel 3 4 1

2 Carbon tetrachloride Dichloromethane µ = 0 D µ = 1.62 D 5 6 Resultant of these two bond dipoles is Resultant of these two bond dipoles is Resultant of these two bond dipoles is Resultant of these two bond dipoles is µ = 0 D Carbon tetrachloride has no dipole moment because all of the individual bond dipoles cancel. µ = 1.62 D The individual bond dipoles do not cancel in dichloromethane; it has a dipole moment. 7 2

3 Assignment 1. Make a three-dimensional drawing of methylamine, CH 3 NH 2, and show the direction of its dipole moment (µ = 1.31D). 2. Ethylene glycol, HOCH 2 -CH 2 OH, has zero dipole moments even though carbon-oxygen bonds are highly polarized. Explain. Acids and Bases There are two frequently used definitions of acids and bases: The Lewis definition The Brøwnsted-Lowry definition 9 10 The Lewis definition A Lewis acid is a substance that accepts an electron pair, a Lewis base is a substance that donates an electron pair Boron in BF 3 has only six electrons in the outer shell and has an empty or vacant p orbital and need a pair of electrons to complete its octet. On the other hand, oxygen in diethyl ether has 2 pairs of unshared electrons. Therefore, BF 3 (Lewis acid) and diethyl ether (Lewis base) can react together to give an acidbase complex

4 The transition metals-containing compounds are excellent Lewis acids. Examples: AlCl 3, ZnCl 2, FeCl 3, and SnCl The Brøwnsted-Lowry definition A Brøwnsted-Lowry acid is a substance that donates protons (H + ), a Brøwnsted-Lowry base is a substance that accepts protons. For example: HCl + H 2 O H 3 O + + Cl - The molecule or ion that forms when an acid loses its proton is called the conjugate base. The molecule or ion that forms when a base accepts a proton is called the conjugate acid. Acid Base Conjugate Acid Conjugate Base

5 Acidity and pk a The acidity constant, K a, is usually expressed the as its negative logarith, pk a. There is an inverse relationship between the magnitude of the pk a and the strength of the acid. The larger the value of the pk a, the weaker is the acid. For example, acetic acid (pk a = 4.75) is weaker acid than trifluoroacetic acid (pk a = 0). HCl (pk a = 7) is a far stronger acid than trifluoroacetic acid Relationship Between Structure and Acidity The strength of a Brønsted-Lowry acid depends on the extent to which a proton can be separated from it and transferred to a base. Removing the proton involves breaking a bond to the proton, and it involves making the conjugate base more electrically negative. Weaker bonds are much easier to be broken than stronger ones. Factors affecting the strength of the bond to the proton: Orbital overlap. Electronegativity. Hybridization

6 The Effect of Orbital Overlap Proton acidity increases as we descend a column in the periodic table due to decreasing bond strength to the proton. Because of decreasing effectiveness of orbital overlap between the hydrogen 1s orbital and the orbitals of larger elements in the column. Comparing hydrogen halides with each other, H F is the weakest acid and H I is the strongest. This follows from the fact that the H F bond is by far the strongest and the H I bond the weakest The Effect of Electronegativity Proton acidity increases from left to right in a given row of the periodic table due to: Increasing the polarity of the bond to the proton. Increasing the stability of the anion (conjugate base) that is formed when the proton is lost. For example: CH 3 H H 2 N H HO H F H The Effect of Electronegativity Because fluorine is the most electronegative, the bond in H F is most polarized, and the proton in H F is the most positive. Therefore, H F loses a proton most readily and is the most acidic in this series

7 Because H F is the strongest acid in this series, its conjugate base, the fluoride ion (F ), will be the weakest base. Fluorine is the most electronegative atom and it accommodates the negative charge most readily: Methane Ammonia Water Hydrogen fluoride The Effect of Hybridization The protons of ethyne are more acidic than those of ethene, which in turn are more acidic than those of ethane: Electrons of 2s orbitals have lower energy than those of 2p orbitals because electrons in 2s orbitals tend to be much closer to the nucleus than electrons in 2p orbitals. With hybrid orbitals, therefore, having more s character means that the electrons of the anion will be lower in energy, and the anion will be more stable

8 The sp orbitals of C H bonds of ethyne have 50% s character, those of sp2 orbitals of ethene have 33.3% s character, while those of sp3 orbitals of ethane have only 25% s character. This means, in effect, that the sp carbon atoms of ethyne act as if they were more electronegative than the sp2 carbon atoms of ethene and the sp3 carbon atoms of ethane. Ethyne Ethene Ethane Electrostatic potential maps for ethyne, ethene, and ethane

Chapter 2 Polar Covalent Bonds: Acids and Bases

John E. McMurry www.cengage.com/chemistry/mcmurry Chapter 2 Polar Covalent Bonds: Acids and Bases Modified by Dr. Daniela R. Radu Why This Chapter? Description of basic ways chemists account for chemical