Ochem lecture notes 343 uwm, Lecture notes of Organic Chemistry

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Chapter 1B. Structure & Bonding (continued)

The Structure of Methane

Carbon has 4 valence electrons (

s

2

p

2

In methane (CH

4

), all C–H bonds are identical (tetrahedral)

Ball-and-stick model show atoms as balls and the bonds as rods

3D drawing:

1. The solid line is in the plane2) The heavy wedged line comes out of the plane3) The dashed goes back behind the plane

Space-filling model portray overall molecular size and shape; the radiusof each sphere approximates the van der Waals radius of the atom

Space-filling model

3D drawing

Ball-and-stick model

4

sp

3

Orbitals and the Structure of Methane

sp

3

orbitals on C overlap with

1s

orbitals on 4 H atoms to form four identical

C-H

bonds

bond: Formed by the head-on overlap of two atomic orbitals along a line

drawn between the nuclei.

All single bonds in organic compounds are

bonds

Bond angle: each H–C–H is 109.

°

, the tetrahedral angle.

5

sp

3

Orbitals and the Structure of Ethane

Two

sp

3

orbitals from two C atoms overlap

head-to-head

to form a C-C



bond

Three

sp

3

orbitals on each C overlap with H 1

s

orbitals to form six C–H



bonds

All bond angles of ethane are tetrahedral and are

near, although not exactly at

,

the tetrahedral value of 109.

o

sp

2

Orbitals and the Structure of Ethylene

Two

sp

2

-hybridized orbitals of two C atoms overlap

head-to-head

to form a

bond

Two p

orbitals of two C atoms overlap

side-to-side

to formation a

bond

One

bond and one

bond result in sharing four electrons and formation

of C=C bond

Electrons in the

bond are centered between nuclei

Electrons in the

bond occupy regions are on either side of a line

between nuclei

sp

2

Orbitals and the Structure of Ethylene

1

s

orbitals of four

H atoms form four C-H

bonds with

sp

2

orbitals of four

C atoms

H–C–H and H–C–C bond angles of about 120

C=C double bond in ethylene shorter and stronger than C-C single bondin ethane

10

sp

Orbitals and the Structure of Acetylene

Two

sp

-hybridized orbitals of two C atoms overlap

head-to-head

to form a

bond

Two p

y

orbitals of two C atoms overlap

side-to-side

to formation a

P

y

-P

y

bond

Two p

z

orbitals of two C atoms overlap

side-to-side

to formation a

P

z

-P

z

bond

One

bond and two

bonds result in sharing six electrons between two

C atoms and formation of C

C bond

sp

Orbitals and the Structure of Acetylene

1

s

orbitals of two

H atoms form two C-H

bonds with

sp

orbitals of two C

atoms

H–C–C bond angle is 180

C

C double bond in ethylene shorter and stronger than C-C & C=C

bonds

Summary

Drawing Structures: Condensed Structures

Drawing every bond in organic molecule can become tedious.

Several shorthand methods have been developed to write structures.

Condensed structures don’t have C-H or C-C single bonds shown.They are understood.