2 Lewis structures - Lecture 2 organic chem PDF

Preview

Summary

Lecture 2 organic chem...

Description

Organic Chemistry – Chemistry of Life and Beyond….. Synthesis

Functional Groups

Reactions

nomenclature

Mechanisms electron movement

Molecules formula shape name bonding

Atoms

1

Lewis Structures • Organic chemists are concerned with the electrons in atoms and more importantly, electrons in molecules. • Before we can understand the behavior of electrons in molecules we need to look more closely at the electronic structure of an atom.

2

Lewis Structures • Gilbert N. Lewis • Valence shell: the outermost electron shell of an atom. • Valence electrons: electrons in the valence shell of an atom → these electrons are used to form chemical bonds. • Lewis structure: – the symbol of the atom represents the nucleus and all inner shell electrons. – dots represent valence electrons.

3

Counting Valence Electrons • You can always calculate the number of valence electrons by analyzing the e- configuration. • Or, for Group A elements only, just look at the Group number on the periodic table (Group number = # of valence electrons)

4

Lewis Structures Lewis Structures for Elements 1-18

Mg

5

Lewis Structures In a Lewis structure: – bonding electrons are represented as a single bond (a single line) and nonbonding electrons as a pair of dots (a lone pair). – Single bond è atoms share one pair of electrons, Double bond è atoms share two pair of electrons, Triple bond è atoms share three pair of electrons.

6

RULES FOR EVALUATION OF LEWIS DIAGRAMS 1) Never place like charges (+/+ or -/-) on adjacent atoms if it can be avoided. 2) The total number of charges in the structure should be kept to a minimum. 3) The magnitude of the charges on any atom should also be kept to a minimum. Multiple charges (+2, -3, etc.) should be avoided. 4) Negative charges should be placed on the most electronegative elements whenever possible. 5) Positive charges should be placed on the least electronegative elements whenever possible. 7

Lewis Structures BOTH STRUCTURES FOR H2NCHO FOLLOW THE LEWIS RULES

WHICH ONE IS BETTER?

O

O

H C N H H

-

H C + N H H 8

Lewis Structures ALL THREE STRUCTURES FOR HCNO FOLLOW THE LEWIS RULES

WHICH ONE IS BETTER?

2_

H H H

.. C .. _ .. C

C

+ N + N + N

+ O:

.. O: _ .. O ..:

9

COMMON BONDING PATTERNS FOR THE ELEMENTS (NO CHARGES) I

III H

monovalent no pairs

IV

V

VI

C

N

O

VII Cl

B

trivalent no pairs

monovalent three pairs

C

N

O

C

N

divalent two pairs

C

trivalent one pair

tetravalent no pairs

10

Lewis Structures – common bonding patterns Atoms use these normal bonding patterns in preference to others. Try these first when building Lewis Diagrams. When these normal patterns are used the atoms do not carry “ formal charges” . However, atoms do use other bonding patterns when forming ions – and there is formal charge.

11

Lewis Structures Molecular formula: C3H6O, CHN, C2H4, C2H5NO2 Structural formula: (CH3)2CO, HCN, CH2CH2, H2NCH2COOH The molecular formula gives you the number of each individual atom in a molecule. The structural formula does the same but also gives information on the connectivity of the molecule.

12

Lewis Structures Molecular formula: C3H6O, CHN, C2H4, C2H5NO2 Structural formula: (CH3)2CO, HCN, CH2CH2, H2NCH2COOH

H O H H C C C H H

H

H C N hydrogen cyanide

acetone

H O H C

C

H H ethylene

H

H C

C O

H N

H

H glycine

FOR MANY COMPOUNDS YOU WILL BEGIN TO SEE PATTERNS

13

Practice drawing the Lewis structures of: HCl, H2O, NH3, NH4+, CO32-, C2H4, C2H2, CH3NO2, CO2, N2O, NO3-, C2H5Cl, H2CO, HCN, and BF3.

14

COMMON BONDING PATTERNS IN WHICH AN ATOM CARRIES A FORMAL CHARGE CATIONS

+

.. + O

+

.. O

+

N

C

N

+

N

+

N

+

ANIONS _ B

_ C:

_

..

N:

_

.. N ..

_

.. : O .. 15

COMMON BONDING PATTERNS FOR THE ELEMENTS (NO CHARGES) I

III H

monovalent no pairs

IV

V

C

N

VI O

VII Cl

B

trivalent no pairs

monovalent three pairs

C

N

O

C

N

divalent two pairs

C

trivalent one pair

tetravalent no pairs

16

Lewis Structures

17

Lewis Structures - exceptions to the octet rule Molecules containing atoms of Group 3A elements particularly boron and aluminum: Cl

H B H

H

Al

Cl

Cl

6 electrons in the valence shells of boron and aluminium

18

Lewis Structures - exceptions to the octet rule Group 5A or 6A elements can form expanded-octet structures

O

H

O S O H O Sulfur can form up to 6 bonds

This is because 3d orbitals are available to be filled

19

A COMMON LANGUAGE FOR CHEMISTS Lewis diagrams are one of the principal methods chemists have for communicating structural information to one another. Handbooks use these diagrams to communicate structural information. When you look in the CRC handbook or the Merck Index you will find Lewis diagrams. Lewis diagrams will help you understand the chemistry of molecules and help to identify reactive sites. We use them to explain reactions. 20...