Transcript More on Hydrocarbons

More on
Hydrocarbons
Isomers:
Simple definition: Different compounds with the same molecular formula
Isomers
Constitutional isomers
Stereoisomers
(connectivity differences)
(configuration differences)
Enantiomers
Diastereomers
Geometric
Optically active
Meso
Constitutional or Structural Isomers

So far we have drawn all carbon atoms connected like beads on a
string, but…

What if we rearrange how the carbon atoms are connected or what if
we move double and triple bonds around?

We get constitutional isomers:

same molecular formula but different structures
 different chemical and physical properties
(We can draw them in several ways: structural, skeleton, line.)
Comparison of physical properties of constitutional isomers:
melting point (◦C)
boiling point (◦C)
Alkanes:
C4H10: n-butane
isobutane
-138
-159
0
-12
C5H12 : n-pentane
isopentane
neopentane
-130
-160
-17
36
28
9.5
Alkenes:
C4H8 : 1-butene
cis-2-butene
trans-2-butene
-185
-139
-106
6.3
3.7
0.9
C5H10 : 1-pentene
2-methyl-1-butene
-165
-138
30
31
Consider free rotation of bonds versus
restricted rotation of bonds:
Single bonds: free or restricted?
Double bonds: free or restricted?
Cyclic compounds: free or restricted?
Geometric isomers - alkenes
H
Cl
H
Cl
H
Cl
Cl
H
Two systems to specify geometric isomers:
Trans = opposite ; or cis = together : each double bond C has a
hydrogen and another group attached
E (entgegen) = opposite ; or Z (zusammen) = together : a more
general system
Geometric isomers – cyclic compounds
OH
H
H
OH
OH
OH
H
H
Cyclic compounds also have restricted rotation between carbons in the ring.
Naming organic compounds

We can draw a lot of different compounds now. But what
to call them?

Every organic compound has a unique name.

IUPAC = International Union of Pure and Applied
Chemistry

The IUPAC name for each compound is unique,
descriptive and specific.
IUPAC Rules of Organic Nomenclature

Organic compounds are named very systematically.



From the name, you should be able to draw the compound!
From the compound’s structure, you should be able to write the
name!
Rules for naming hydrocarbons are listed in your text.






p. 309
p. 311
p. 326
p. 340
p. 356
p. 359
alkanes
cycloalkanes
alkyl halides
alkenes
alkynes
aromatics
The Rules for Alkanes, as listed in your text:
1.
The name of the longest chain becomes the BASE, or
parent name, of the compound. The suffix (ending) of
the family name is added to the end of this base name.
Note that, even if the subsequent rules are correctly
applied, a failure to correctly identify the longest chain
will result in an incorrect name for the compound.
2.
The base name accounts only for the carbons in the longest chain. The
carbons that are not part of the longest chain – those attached as
branches to the longest chain and called substituents or groups - must
also be included in the name. Substituents are included as follows:
A. The name(s) of any alkyl group(s) in the compound is placed in front
of the base name.
B. Use the prefixes di, tri, tetra, penta, and hexa, before the name of the
alkyl group when there are two, three, four, five, or six, respectively, of
the same group.
C. alphabetize the names of alkyl groups when there are two or more
different types of groups. Ignore all prefixes (both the branching prefixes
s- and t- and the multiplying prefixes such as di-, tri-, and tetra-) in
alphabetizing, with one exception: iso- is not ignored in alphabetizing.

3. Number the carbons in the longest chain, starting from whichever end
will result in the lowest number (or set of lowest numbers) for the alkyl
group(s). An alternate rule is useful for most compounds: number from the
end nearest a branch.

4. In front of the name of each alkyl group, place the number of the carbon
to which the group is attached.

5. Use hyphens to separate numbers from words; use commas to separate
numbers.
More on bonding: Hybridization

s bonds– interactions between two s-orbitals of
two atoms
 Geometry

p bonds – interactions between two p-orbitals of
two atoms
 Geometry

of s-orbitals?
of p-orbitals?
Orbital hybridization
 sp3
 sp2
 sp
Or electron density looks like
this:
Benzene and Aromatic compounds
How can you form a molecule with 6
carbons and 6 hydrogens fulfilling all
bonding requirements for each atom?
Some proposed structures:
(They seemed reasonable enough at the time)
August Kekule and his dream
“I was sitting writing at
my text book, but the
work did not progress;
my thoughts were
elsewhere. I turned my
chair to the fire and
dozed. Again the atoms
were gamboling before
my eyes. …But look!
What was that? One of
the snakes had seized
hold of its own tail, and
the form whirled
mockingly before my
eyes….Let us learn to
dream, gentlemen, and
then perhaps we shall
learn the truth.
We draw benzene like this and say
the electrons in p-orbitals are
delocalized:
p-orbitals are
perpendicular to
the plane of the
molecule. Does it matter
where you draw the double
bonds in this case?
How do we prove this is correct for benzene’s
unique structure?
From X-ray crystallography, we can
determine the following for carboncarbon bond lengths:
Single bond length: 1.47 A
Double bond length: 1.33 A
C
C
C
C
C
C
Benzene C-C bond lengths
are all: 1.39 A
Hmmm.
Fancy words for this:
Electron delocalization
And aromaticity
Some aromatic structures to recognize (associate
name with them)
1.
2.
3.
4.
5.
Benzene
Phenol
Toluene
Aniline
Benzoic acid
OH
phenol
O
CH3
toluene
NH2
aniline
C
OH
benzoic acid
Other aromatic structures:
Polycylic aromatics and heteroatomic
aromatics
P h en an th rene
N ap h th alen e
B u ilding blo cks of D N A , R N A :
H
N
N
N
N
N
A nthracene
N
P urine
P yrim idine
Note on Definitions
Aliphatic
Aromatic
What do you think about the
electrons in a situation like this?




Consider overlap of p-orbitals:
Conjugated?
Delocalized?
Aromatic?
H 3C
CH3
CH3
CH3
OH
V itam in A (trans-retinol)
CH3
What about the electrons in this molecule?
Overlap of p-orbitals:
Conjugated?
Delocalized?
Aromatic?
CH3
CH3
H 3C
CH3
CH3
CH3
O
CH3
HO
V itam in E ( -to co p h e ro l)
CH3
What about these electrons:
Overlap of p-orbitals:
Conjugated?
Delocalized?
Aromatic?
O
CH3
O
V itam in K 2