Organic Chemistry - Paula Bruice - 6th ed 2011

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A polar covalent bond has a slight positive charge on one end and a s light negative charge on the other. Polarity in a covalent bond is indicated by the symbols S+ and S-. which denote partial positive and partial negative charges. respectively. The negative e nd of the bond is the end that has the more electronegative atom. The greater the difference

tontc and Covalent Bonds in electroneglltivity between the bonded moms, the more polar the bond will be. (Notice that a pair of shared electrons can be shown as a line between two atoms.) ~

~

~

H-O:

~

~

H-N-H

~

h

The direction of bond polarity can be indicm ed with an arrow. By conve ntion, chemists draw the arrow so that it points in the direction in which th e el ectrons are pulled. Thus, th e head of the arrow is at the negative end of the bond; II short perpendicu lar line near the tail of the arrow marks the positive e nd of the bond. (Physicists draw th e arrow in the opposite direction.)

You can think of ionic bonds and nonpolar covalent bonds as being at th e opposite ends of a continuum of bo nd types. All bonds fall somewhere on this lin e . At one e nd is an ionic bond, a bond in which there i. no sharing of electron •. At th e other end i. a nonpolar covale nt bond, a bond in which the electrons are shared equally. Polar covalent bonds fall somewhere in between. The greater the diffe re nce in electronegativity betwee n the monts forming the bond, the closer the bond is to the ionic end of the continuum.

continuum of bond types

,.,1. covalent bond

ionic boo' K'" P- N.+CI-

O-H

1

co,..~""'. alent bond

N-H

C-H C- C

C - H bonds are relatively nonpolar, because carbon and hydrogen have similar electron egativities (electronegativity difference = 0.4; see Table 1.3); N - H bonds are more polar (electrone gativity differe nce = 0.9). but not as polar as O- H bonds (electroneglltivity difference = 1.4). Even closer to th e ionic end of the continuum is the bond betwee n sodium and chloride ions (clectronegativity difference = 2.1), but sodium chloride is not as ionic as potassium fluoride (el eetronegativity difference = 3.2). PROBLEM 6.

n. H-CH]

Which bond i. more polar? or Cl - CH]

c. H-Cl or H-F

b. H-OH or H-H

d. Cl - Cl or Cl- CH]

_boOO~.

PROBLEM 7.

Which of the following h", the most polar bond? Nal LiBT CI 2

n.

QIUI11 Tmorilll:

pI..-a EIect,ooeqariYity di_~.

h. the least polar bond? KCJ

A polar bond has a dipol e--it has a negative end and a positive end. The size of the dipole is indicated by the dipole moment , symbolized by the Greek letter po. Th e dipole monlt'nt of /I bond is equal to the magnitude of the charge (e) on the atom (either the partial positive charge or the panial negative charge, because they have the same magnitud e) times the distance between the two charges (d): dipole mome nt

=

po

=

e Xd

11

12

CHAPTER l ' Electronic Strudure and Bondlng ' Acldsand Bases

BIOGRAPHY

A dipole moment is reponed in a unil called a dehyt' (D) (pronounced de-bye). Beca use the charge on an electron is 4.80 X 10- 10 eleclroslatic units (esu) and Ihe dislance between charges in a polar bond will have units of IO-Il cm. th e produci of charge and distance will have units of 10- 18 esu cm. Thus. 1.0 D = 1.0 X 10- 18 esu cm. so a dipole momem of 1.5 X lO- t8 esu cm can be more simply stated as 1.5 D. Th e dipol e moments of some bonds commonly found in organic compounds an: listed in Table 1.4. Table 1.4 The Dipo le Moments of Some Commonly Encountered Bonds Dipole IlI on",nt (D )

Bond

HHHHH-

Pftw Dl>bYIl (1884-1\166) IonS born in the Netherlands. H~ lallght at lhe Uni ....,rsiriu o/Zurich (suecuding Einstein). u,ipzig. and B~rlin. bul rnllmed 10 itis ltom~land in /l).il) lI'hen Ihe Na;p ordered him 10 ~come a G~rman eiti-, ............. -1.

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U (-- sp" CH, CHJ - NH2

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w'

CH]- C_ N:

CHj

CH]- RH

CH]

:p - C- Q:

-

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1 1 1 w'

w w

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In comparing the lengths and strengths of cllrbon---carbon singl e. double. and triple bonds. we see thm th e more bonds holding two carbon atoms together. the shorter and stronger is the carbon-earbon bond (Table 1.7): tripl e bonegativities given in T.ble 1.3): II.

H~ - Br

l~

H,C- Li

e. HO - NH1

d. ] - 8 r

c. HlC - O H

82. What is the hybridi:tation of the indicated atom in each of the following molco;ules?

1

II. CHf: " - CH l

1

.... Clif:H ~H

0h. CH1 D-lJ

d.

1

CH ~ _ N

1

o. CHf: .I - NC H3

r.

1

D-Itx:H2CH]

f. (CH lhN - H

Problem s

6S

83. DllIw the missing lone-pair clC!l'trons and assign the miaing formal chlllJeS. H II.

H-f-O- H H

84.

II .

H l>.

H

H-{-crH H

c.

H- ?-O

H

H

List the following carboxylic acids in order of decn::a~ing acidity:

C I1 3CH1Cli~OOH

IC. _ 1. 52 x 10-"

C ll lOll~:iCOOH

ClC H1ClI 1Cli~OOH

C H1:I CIi~OOH

IC. _ 2.9i x 1n-'

IC. _ l.l9 x 11i·

K. _ 1.9 x 10-"

b. How doe.. the presence of WI clcclronegative substituent such as Cl affect the acidity of a carb",.ylic acid?

c. Haw d~ the location of the substituent arrect the acidity of the carboxylic acid? 85. Draw the produelSof the foll owing reactions:

86.

Whic h of the indicated bonds in each molecule is shaner? b. Indicute the hybrid ization of the C. O. and N atoms in each of the moIC!l'wt'5.

II.

87. List the following compounds in ordoo r of decreasing acidity.

88. For each of the following cOOlP'Junds. draw the form in which it will predom inate at pH '" ), pH "" 6. pH '" 10. and pH "" 14: II,

CHf:OOH

P«. - ....

b. CH lCH1NH) pIC. _ 11,0

89. Do the spl carbons and the indicllted spl carbo ..... lie in the same plane?

90. Give the products of the 101i0wing acid--base reactions. and indicate whether re""taru.. or pnxIucu are favored at eq uili bri um (usc the pK~ valu,"" that an: given in Section 1.18):

66

CHAPTER 1 · Electronlc Siructure and Bonding' Acid s a nd Ba SI!5

9 L a. L isc che follOYYing a1robols in order o f decreasing llcid icy: h. Explain che relative aciditie..

92. Which of the following sp