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Orgo: Molecular Structure
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Chemistry
04/27/2009

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Term
Lewis Dot Structures
Definition

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)

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

does not show bonds

 

CH3CH2CH2OH

Term
Bond-line formula
Definition
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
Term
Fischer projection
Definition
verticle lines are assumed to be oriented into the page; horizontal lines are assumed to be oriented out of the page
Term
Newman projection
Definition
is a view straight down the axis of one of the σ-bonds; both intersecting lines are assumed to be Cs
Term
dash-line-wedge formula
Definition

black wedge = coming out of page

dashed wedge = going into page

lines = in plane of page

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

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

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

Prefix            # of Cs

meth-                1

eth-                   2

prop-                 3 

but-                   4

pent-                 5

hex-                  6

sept-                 7

oct-                   8

non-                  9

dec-                  10

Term

Bonding

 

Definition
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
Term
Coordinate covalent bond
Definition
one nucleus donates 2 (both) e's to form a bond
Term
σ bonds
Definition

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

Term
∏ bonds
Definition

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

2) orbitals form above and below the σ-bonding e's (double bond)

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

4) independently weaker than σ-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

Term
hybridization
Definition

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