1.5   Basic Chemistry background

A)   Bond     The lewis theory about chemical bonding:
    a) valence electrons (outmost electrons) play a fundamental role in chemical bonding
    b) When metals and non metals combine, valence electrons are usually transferred from the metal atoms
        to the non metal atoms. Cations and anions are formed and electrostatic forces for attractions between
        the ions give rise to ionic bonds.   For example:  2Li + F₂ --> 2Li⁺F^-
    c) In combinations involving only nonmetal atoms, one or more pairs of valence electrons are shared
        between the bonded atoms, producing covalent bonds. For example, CH₄ are compounds in which
        H atoms are bonded to C atoms through covalent bonds.
    d) In losing, gaining, or sharing electrons to form chemical bonds atoms tend to acquire the electron
        configuration of noble gases, (Octet rule),
        i.e. 1st shell 2 electrons, 2nd shell 8 electrons, 3rd shell 18 electrons

b) sp² orbitals (the others are sp3 and sp orbitals)
    For example: Ethylene, C₂H₄ (CH₂=CH₂)

The best explaination of the unique structure of ethylene is sp² orbitals theory.

sp² orbitals of two carbons form a s bond, and the parallel p orbitals of two carbons form a p bond.

    Practice problem 1.9  Draw all the bonds in propene, CH₃CH=CH₂, indicate the hybridization of
    each carbon, and predict the value of each bond angle.
    ( Tip: the best way to assign which orbitals is to find how many s bonds)

C) Drawing Chemical Structures

D) Organic Compounds and properties

Functional Groups: a group of atoms that has about the same chemical properties
whenever it occurs in different compounds.

1. Carbon-Carbon Multiple Bonds.
2. Carbon singly Bonded to an Electronegative Atom
3. Carbon-Oxygen Double Bond (Carbonyl Groups)

R- group, unless otherwise indicated, stands for any carbon group (Rest of the molecule).
R-X: Alkyl halide, e.g. CH₃-Cl, (CH₃)₂CH-Br.
Alkyl groups are derived from alkane. Aryl groups are derived from benzene.

Some common Functional Groups:

                                               

	Functional Group Structure	Simple example	Property
Alkane	C-C	CH3CH3	Nonpolar, stable, low mp and bp
Alkene	C=C	CH2=CH2	Nonopolar, low mp and bp, reactive
Alkyne	C=C	CH=CH	Nonpolar, low mp and bp, reactive
Arene		benzene	nonpolar, high mp and bp, stable
Halide	R-X
Alcohol	R-OH
Ether	R-O-R'
Amine	RNH2
Nitrile	C=N
Aldehyde
Ketone
Carboxylic acid
Ester
Amide

Others are: Nitro, sulfide, sulfoxide, sulfone, thiol, acyl halide, anhydride.

E) Overview of Organic Reactions

Four kinds organic reactions -- Additions, eliminations, substitutions, and rearrangements.

Addition reactions:         A + B -->  C                     Example: CH₂=CH₂ + HBr --> CH₃CH₂Br
Elimination reactions:      A --> B + C                   Example: CH₃CH₂Br --> CH₂=CH₂ + HBr
Substitution reactions:     A + B -->  C + D            Example: CH₄ + Cl₂ --> CH₃Cl + HCl
Rearrangement reactions:   A -->  B                      Example:  such as "smart materials"

Mechanisms of Organic Reactions:                                   and                                         .

All chemical reactions involve bond breaking and new bond formation.
Two ways in covalent bond breaking-homolytic and heterolytic.
Homelytic-symmetrical cleavage, produce two radicals. (half headed arrow)
                    e.g. Cl₂ --> Cl· + Cl·
Heterolytic-usymmetrical cleavage, produce a positive charged fragment and a negative
                    charged fragment.  (full-headed arrow)
                    e.g.   HCl --> H⁺ + Cl^-
Or course, two ways in covalent bond formation-homogenic and heterogenic.
Homegenic-electrons symmetrical donation from two radicals
Heterogenic- electrons usymmetrical donation from only one reactant.
 

Radical reactions:
A radical generally has 7 electrons in its valence shell.

Addition.       Rad ·  + CH₂=CH₂--> Rad-CH₂-CH₂ ·

Elimination.    Cl:Cl --> Cl· + Cl·

Substitution.      Rad ·  + A:B --> Rad:A  +  · B

Polar reaction.
Polar reaction are strongly related to the electronegativity of atoms in a molecule.
Electron-rich site (d-) in one molecule react with electron-poor site  (d+) in another molecule
in all polar organic reactions.

A carbon atom bonded to N, O,  F, Cl, Br, I atoms always has a partial positive charge (d+).
A carbon atom bonded to a metal has a partial positive charge (d-), such as Grignard
            reagents and alkyllithium, alkylcalcium, etc.

When drawing a curved arrow: The electrons (two electrons) move from the atom or bond
(at the tail of the arrow) to the atom (at the head of arrow).
For instance:

Electron-poor substance is called electronphile (electron-loving)

    For instance:    H⁺   C in CH₃Br,   C in R₂C=O,  , etc.
Electron-rich substance is called nucleophile (nucleus-loving)
                                                  
    For instance:    CH₂=CH_2, H₃N:,    H₂O: ,   OH^- ,   Br^- ,  etc.
H₂O, ROH, RCOOH, etc. can be either a nucleophile or an eletronfile, depending on the
which part react with other molecules.

Rules on curved arrows and samples:

Rule 1. Electron pair (two electrons) move from a nucleophilic molecule (Nu:)
            to an electrophilic molecule (E)
            Nucleophile can be either negatively charged, such as CH₃O^-, Br^-, Cl^-, etc.,
                                          or neutral, such as CH₂=CH₂, H₂O, etc.
            Electrophile can be either positively charged, such as R₃C⁺, CH₂=OH⁺, H⁺,
                                          or neutral, such as C (d+) in CH₃Cl, C(d+) in R₂C=O, etc.
Rule 2. The octet rule must be followed.

Homework questions

         1. What is the formula of the following compounds?

2. Follow the flow of electrons indicated by the curved arrows in the following reaction,
      which product will be produced?

    A) CH₃CH=CH₂ + A^-    B) CH₃CH₂CH₂⁺   +  A^-     C)  CH₃CH⁺CH₃   +   A^-
    D) CH₃CH(A)CH₃      E) CH₃CH₂CH₂A

3. Which of the following is not a nucleophile?
    A) H₂O     B) CH₃O^-    C) NH₃     D) NH₄⁺    E) All are necleophile

4. Which of the following is a substitution reaction?
    A) CH₃CH₂Cl + I^- --> CH₃CH₂I + Cl^-    B) CH₃CH₃ + Cl₂ --> CH₃CH₂Cl + HCl
    C) CH₃CH₂Cl  -->  CH₂=CH₂ + HCl
    D) More than one of the above  E) None of the above