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O CHEM MCAT

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0hemistPatt ISection IStructure, Bonding, and Reactivity

Organ

Sections I-IVSection IIStructure Elucidation

Section IIIStereochemistry

$ection IVHydrocarbon Reactions

'll'te

K.lE. V. I. E. W Speci alizing in II{CAT Preparation@

Nomenclature

a) b)

IUPAC Nomenclature

General Nomenclature

Bonding and Molecular Orbitals

Section IStructure, tsonding,and

a) Lewis Dot Structures b) Bonding Model c) Covalent tsonds d) Molecular Orbitals and Bondse) g) a)b)

i. ii. iii.

f)

Single Bonds Double Bonds Triple Bonds Molecular Structures Octet Rule (HONC Shortcut) Charged Structures Hybridization of Atomic Orbitals sp-Hybridzation sP2-HYbridzation sP5-nybridzation Common Shapes

Hybridization

Reactivity

" double bond > single bond. Another rule to consider is that for bonds between like atoms, the ionger the bond, the less the electron density overlaps between nuclei, and thus the weaker the bond. This is summarized as: longer ionds are rueaker bonds.

of

single Bonds: single bonds are composed of only one sigma bond between the two atoms. single bonds are longer than double and triple bonds between two atoms, even though fewe_r electrons are present. Ethane has sigma bonds only and is shown in Figure 1-8 in both stick figures and with the rele')ant orbitals.

HH

H

H

HH

/v"C-C.

OC

ffi"porp3-5p3

/H

\"

Figure 1-8 Double Bonds: Double bonds are composed of one sigma bond and one pi bond between two adjacent atoms. Ethene (czH+) has f6ur sigma bonds between carbon and hydrogen, a sigma bond between the two carf,ons, and a pi bond present between the two carbons to compiete the carbon-carbon double bond. Ethene is shown in Figure 1-9 in both stick figures and with the relevant orbitals.ftp^-p^

Illtt,,,

H7:

\n

..rr\Hosp2-roz

H

%o="# @rffi%

Figure 1-9

Triple Bonds: Tripie bonds are composed of a sigma bond. and two pi bonds between two adjacent atoms. Triple bonds are shorter than either iingte or double bonds. Ethyne (CzHz) has two sigma bonds between carbon and hydrogen, a sigma bond between the two carbons, and two pi bonds between the two carbons to complete the carbon-carbon triple bond. Ethyne is shown in Figure 1-10 in both stick figures and with the relevant orbitals.fipv-pv

H

@.- .P

Figure 1-10

Copyright O by The Berkeley Review

lo

The Berkeley Review

Organic ChemistryExample 1.4

Molecular Structure

Bonding and Orbitals

\A/hat is the relative bond strength of carbon-carbon bonds in the molecule shown

below?

bondaS\bond c

,rr..

ooloo

_-_*/

l, / C*C\

,r,-\ z .v ^CH^CH"rC CHe

H

(H3.)2HC

""Y TA. Bond a > Bondb > Bond c > Bond d B. Bondb > Bond a > Bond c > Bond dC. D.Bond d > Bond a > Bond c > Bondb Bondb > Bond c > Bond a > Bond d

bond d

SolutionThere strongest C-C bond is a double bond, bond b, so choices A and C are -Bond eliminated. d is the weakest, because it is between two sp3 carbons. Bond c is stronger than bond a, despite both sharing an sp2 and an sp3 carbon, because bond c contains the more highly substituted carbon. Choice D is the best answer.

Molecular Structures We shall continue from the fundamental concept that a valence electron can be shared between two nuclei rather than being isolated to just one nucleus, because the attractive force of two positive sites is greater than the attractive force of one. This is the basic, perhaps oversimplified, perspective of a chemical bond. The sharing of electrons is what characterizes a covalent bond. One of the first rules of organic chemistry that you must understand is the octet rule. It is valid for carbon, nitrogen, and oxygen atoms. To understand organic chemistry, it is important that you recall VSEPR theory, which applies to bonding (in particular, to the subgroups of covalent bonding like single, double, and triple bonds and their component o-bonds and n-bonds). Table 1-4 shows the skeletal structures of molecules that contain carbon, nitrogen, oxygen, and hydrogen. AtomValence Electrons To Complete Shell4 e- needed

Number of Bonds in Neutral Compounds

Carbon (C)

4. 5.b

.c..N.

n3

I

-tY'_-6-_

-L_

Nitrogen (N)

3 e- needed

'"; - -''Y', \ -N'

i5:

Oxygen (O) Hydrogen (H)

- .ct...

2 e- needed1 e- needed

,

:O.

1H.

1H-

Table 1-4

Copyright

@

by The Berkeley Review

ll

Exclusive MCAT Preparation

Organic Chemistry

Molecular Structure

Bonding and Orbitats

(

Octet Rule and the HONC Shortcut: Every molecular structure should have atoms that obey the octet rule (eight valence electrons for C, N, and o). The numbers of electrons needed to complete the shell in the Table 7-4 are derived from the electrons needed to obey the bctet rule. All neutral structures have atomic arrangements as described inTable 1.4. To complete the octet valance shell, carbon requires four electron pairs in the form of bonds, nitrogen requires one lone pair in addition to the thiee bonds it makes, and oxygen requires two lone pairs in addition to the two bonds it makes. You must be able to recognize valid structures by applying the bonding rules (HONC-1234). In a neutral compound, hydrogen makes onebond., oxygen makes truo bonds, nitrogen makes threebonds, and carbon makes four bonds. Neutral structures always obey this rule. Figure 1-11 shows examples of valid and invalid structures and a brief description of the bonding to the component atoms.

C:

-.{j

::

:,:

::.

r)-l

-a

'-'

T

tt/\ HHL-L

/tnt

H \///\ HH^-A L-L

,CHZ

All carbons have 4 bonds. All hydrogens have 1 bond.Good Structure

Most carbons have 4 bonds, but one carbon has 5 bonds. All hydrogens have 1 bond.Bad Structure

HsC- C- CAll

CH2CH2CH3

H"C-C:

/CH3

carbons have 4 bonds.

cH2cH3 AII carbons have 4 bonds. Ali hydrogens have 1 bond, but oxygen has 3 bonds.Bad Strttctttre

A1l hydrogens have 1 bond.Good Structure

Dn:-..!itl__

__

:_

H '\1\-L

/-"3

CH"

tra, N-CHA11

CHs

:_r

\

CHs

H

Ali carbons have 4 bonds.A1l hydrogens have 1 bond. Nitrogen has 3 bonds.Good Structure

All carbons have 4 bonds.hydrogens have 1 bond, but nitrogen has 4 bonds.Bad Structure

Figure 1-11

You can validate molecular structures by seeing whether they satisfy bonding rules (HoNC-7234) and conventions with regard to the number of bonds and lone pairs. If a structure does not satisfy the rules, then there must be a charge present. Generally, having too many bonds in a molecule results in a cation and too few bonds results in an anion, except with carbon. For instance, if oxygen makes three bonds and has one lone pair, it carries a positive charge. wnu.t nitrogen makes two bonds and has two lone pairs, it carries a negative charge. \Alhen carbon makes three bonds, the charge depends on the presence or absence of a lone pair (presence yields an anion, r,t'hile absence yields a cation). Copyright@

by The Berkeley Review

t2

The Berkeley Review

-i!

Organic Chemistry

Molecular Structure

Bonding and Orbitals

Charged Structures Formal charges (charged sites on a molecule) occur when there is an excess, or shortage of electrons on an atom. For instance, an oxygen atom typically has six valence electrons and wishes to have eight. This means that oxygen makes two bonds to complete its valence shell (and thus satisfy the octet rule). However, if an oxygen atom had only five valence electrons, it would be short one electron from its original six and would consequently carry a positive charge. Having only five valence electrons, the positively charged oxygen would need to make three bonds (one more than its standard two) to complete its octet. We can conclude that oxygen with three bonds carries a positive charge. Table 1-5 shows somecommon organic ions to commit to memory: Number of Bondsto Neutral Atom

Number of Bonds to Cationic Atom

Number of Bonds to

Anionic Atom

Carbon (C)

43

.c.4t'

3l-/t\

? ..-+ r+-

-'Y',-\/-1--

Nitrogen (N)

t

'N'

z\':N\-5+=

-t-

Oxygen (O)

2

.o.

'-ij\,:t

:o*=

t

-p]'

Table 1-5

Drawing Molecular Structures Drawing molecular structures from a given formula requires following the octet rule for aII atoms except hydrogen. On occasion, thete will be charged atoms within the compound, but the number of charged atoms within the structure should be minimized. Figure 1-12 shows some samPle structures for a fewcommon molecules.

Molecular and Structural Formula

Lewis Structure

3D Structure

c2H60 cH3cH2oH

HH ll H_C_C- Otl HHHHH

HHH

.C_C \t\ H.;H

liu \it..v II ^- rr

U

c2H7N cH3NHCH3

H_ C-N_ C_ H

rtl

HHc2H50* cH3cHo+H

t..l

: C_ N' .( )Y HtH .C_HH

H

ll H-C-C:H

HH

"irH.C-C \\.g*-H

t..

O*- H

"'*l5

Figure 1-12 Copyright@

by The Berkeley Review

Exclusive MCAT Preparation

Organic Chemist