Orgo: Molecular Structure

Lewis Dot Structures

Most basic form of representing molecular structures.


Rules to forming LDS:

1) find total # of ve’s for all atoms in molecule

2) use one pair of e’s to form one bond between each atom

3) arrange remaining e’s around the atoms to satisfy the duet rule for H and

;;; the octet rule for other atoms


Exceptions:

1) compounds with B or Be may contain less than an octet

2) Molecules with an atom containing more than an octet MUST contain an

;;; atom from the 3rd period or greater in the PT (d orbitals)

Formal charge
# of e’s in isolated atom – # of e’s assigned to the atom in the Lewis structure
Dash formula
shows bonds between atoms, but does not show 3D structure of molecule
Condensed formula

does not show bonds

;

CH3CH2CH2OH

Bond-line formula
line intersections, corners, and endings represent a C atom unless some other atom is drawn in; H atoms attached to the Cs are not usually drawn but are assumed to be present
Fischer projection
verticle lines are assumed to be oriented into the page; horizontal lines are assumed to be oriented out of the page
Newman projection
is a view straight down the axis of one of the ;-bonds; both intersecting lines are assumed to be Cs
dash-line-wedge formula

black wedge = coming out of page

dashed wedge = going into page

lines = in plane of page

Ball and stick models
covalently bonded atoms are drawn to scale using comparisons of their atomic radii as single atoms
Index of hydrogen deficiency

indicates #s of pairs of Hs a compound requires to be a saturated alkane


2n + 2 hydrogens (n = # of Cs)


Index of H deficiency =((2n + 2) – (# Hs in cmpd))/2

;

* Count halogens as Hs

Ignore O atoms

Count N as 1/2 H

Functional Groups
reactive, non-alkane portions of molecules
Nomenclature for alkanes

Prefix;;;; ; ; ;; # of Cs

meth-;;;;;;;;;;;;;;; 1

eth-;;;;;;;;;;;;;;;;;; 2

prop-;;;;;;;;;;;;;;;; 3;

but-;;;;;;;;;;;;;;;;;; 4

pent-;;;;;;;;;;;;;;;; 5

hex-;;;;;;;;;;;;;;;;; 6

sept-;;;;;;;;;;;;;;;; 7

oct-;;;;;;;;;;;;;;;;;; 8

non-;;;;;;;;;;;;;;;;; 9

dec-;;;;;;;;;;;;;;;;; 10

Bonding

;

electrostatic forces between e’s and nuclei create all molecular bonds; both nuclei tug on both e’s and the result is a bond between the 2 nuclei
Coordinate covalent bond
one nucleus donates 2 (both) e’s to form a bond
σ bonds

1) forms when the bonding pair of e’s are localized directly between the two

;;; bonding atoms


2) have the lowest E and are most stable form of covalent bonding


3) strong

; bonds

1) an additional bond formed between 2 ;-bonded atoms

2) orbitals form above and below the ;-bonding e’s (double bond)

3) orbitals form on the sides of the σ-bonding e’s (triple bond)

4) independently weaker than ;-bond (less E required to break the bond)

5) nevertheless, they strengthen overall bond b/t atoms, shorten the bond

;;; length, and increase bond E

6) e’s are further from nucleus than those in σ-bond, thus they are at a

    higher E level, are less stable, and are more reactive

7) 3rd row elements form weaker ∏ bonds than 2nd row elements

8) only C, N, O, and S commonly form double and triple bonds

9) ∏ bonds prevent rotation

hybridization

combined orbitals that create new shapes and E levels:


Hybridization        Bond angles                                 Shape

       sp                  180                                  linear

       sp2                     120                            trigonal planar

       sp3                   109.5                   tetrahedral; pyramidal; bent

     dsp3                 90; 120      trigonal-bypyramidal; seesaw; t-shaped; linear

   d2sp3                 90           Octahedral, square pyramidal; square planar

 

*shape differences depend upon # and location of LPe’s

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