Transcript Ozone/CFCs

Chapt 21 Hydrocarbons [Selected]
21.1 Introduction to Hydrocarbons
21.2 Alkanes [Straight-Chain Only]
21.3 Alkenes & Alkynes (added)
21.4 Hydrocarbon Isomers [also with O]
Section 21.1 Introduction to Hydrocarbons
Hydrocarbons are carbon-containing organic
compounds that provide a source of energy and raw
materials.
• Explain the terms organic compound and organic chemistry.
• Identify hydrocarbons
• Recognize the different ways that hydrocarbon molecules
may be represented (molecular formula, structural formula,
ball-and-stick model, etc.) and convert a molecular formula
into a valid structural formula and vice versa,
• Distinguish between saturated and unsaturated
hydrocarbons.
Section 21.1 Introduction to Hydrocarbons
Key Concepts
• Organic compounds contain the element ??
• Hydrocarbons are organic substances composed of carbon
and ??.
• The major sources of hydrocarbons are petroleum and
natural gas.
Organic Compounds
Name used because living organisms
known to contain/produce them
Term applied to all carbon-containing
compounds except for a small number of
compounds considered to be inorganic
Because huge number exist, an entire
branch of chemistry – organic chemistry –
devoted to their study
Essential Organic Chemistry
Bond – force that hold atoms together in
compounds
Carbon atom (C) always forms 4 bonds with
other atoms; bond represented by a line
Hydrogen can only form one bond
Organic Compounds - Hydrocarbons
Simplest organic compounds; consist of only
the elements carbon (C) and hydrogen (H)
In hydrocarbon, C either attached (bonded)
to another C or to a hydrogen
Are thousands of hydrocarbons; can be in
chain, branched chain, ring, and cage-like
structures
Major source of hydrocarbons – petroleum
and natural gas (mostly CH4 – methane)
Hydrocarbons
Carbon atoms bond to each other by single,
double, & triple bonds (always 4 total bonds)
Saturated hydrocarbons contain only single
bonds
Unsaturated hydrocarbons contain at least
Bond to some
one double or triple bond
unspecified
atom
Single
Double
Triple
Simplest Hydrocarbon - Methane
Chemical (molecular) formula: CH4
Structural formula:
H
Chemical bond
H
Carbon atom with 4 bonds
C
H
H
Simple Hydrocarbons - Methane
One carbon atom attached to 4 hydrogens
Shape of carbon bonded to 4 other atoms is
a tetrahedron – bond angles of 109.5
Hydrogens occupy corners of tetrahedron
109.5
Ways of Representing Compounds
Compounds may be represented by various
types of formulas and graphical presentations
Variety of these shown on following slides
Chemists use form that best shows
information they wish to highlight
Molecular formula most compact but no
information about connections & geometry
Structural show connections but no 3D info
Most graphical forms can be generated and/or
manipulated using online chemical software
Ways of Representing Methane
H
Chemical (molecular) formula CH4
Structural
formula with
additional
geometry
information –
solid wedge
coming toward
you, dashed
one away
H
Structural
formula
Ball &
stick
model
Spacefilling
model
C
H
H
Chapt 21 Hydrocarbons [Selected]
21.1 Introduction to Hydrocarbons
21.2 Alkanes
21.4 Hydrocarbon Isomers [also with O]
Section 21.2 Alkanes
Alkanes are hydrocarbons that contain only
single bonds.
• Name a straight-chain alkane from its molecular formula or
by examining its structure (up to octane).
• Draw the structural formula or write the molecular formula
of a straight-chain alkane when given its name (up to
octane).
Section 21.2 Alkanes
Key Concepts
• Alkanes contain only single bonds between carbon atoms.
• Alkanes and other organic compounds are best represented
by structural formulas and can be named using systematic
rules determined by the International Union of Pure and
Applied Chemistry (IUPAC).
• Alkanes that contain hydrocarbon rings are called cyclic
alkanes.
Alkanes
Simple Alkanes – hydrocarbons with only
single bonds and no ring structures
All have formula CnH2n+2
n = integer
All have names ending in “ane”
Simplest = methane
Chemical (molecular) formula: CH4
Chemical bond
Structural formula:
H
Carbon atom with 4 bonds
H
C
H
H
Alkanes
n = 2 ethane
Chemical (molecular) formula: C2H6
Structural formula:
n = 3 propane
H
H
H
C
C
H
H
Chemical (molecular) formula: C3H8
H
H
Structural formula:
H
H
H
C
C
C
H
H
H
H
“Constructing” Alkanes Stepwise
Can think of alkanes larger than methane
as being built from smaller molecules by
adding a methyl group: CH3
Process: 1) Remove H atom (leave bond)
2) Replace removed atom with CH3
If start with CH4, four possible choices for H
to remove, but all choices result in exactly
the same molecule, ethane = C2H6
Making Ethane (C2H6) From CH4
methane
CH4
H
C H
H
H
H C H
H
ethane
C 2H 6
methyl
group:
CH3
Ethane
As represented by
skeletal formula
Can write formula as C2H6 or as CH3CH3
Latter method allows one to visualize and draw
structure more easily
Additional very compact representation possible –
skeletal (aka line-angle or bond line) formula
Bonds are lines (as before)
Carbon atoms present where line begins or ends
or where 2 lines meet
H not shown unless attached to drawn atom
Ethane
Most alkanes rotate freely about the single bond
between carbon atoms
Single Bond Free Rotation
Free rotation occurs about single bonds
Consequence of free rotation: 2 molecules that
may appear different when drawn may in fact
be identical because one molecule may be
twisted about its single bonds to have the
exact same shape as the 2nd molecule
Two Equivalent Butane Molecules
Right hand structure is twisted
version of left hand structure
Single Bond Free Rotation
Free rotation occurs about single bonds
Because of free rotation all six hydrogen
atoms in ethane are equivalent
If making new compound from ethane by
replacing a hydrogen, doesn’t matter which
one is chosen – result will be the same
Making Propane (C3H8) From Ethane
Ethane: C2H6
H H
H C C H
H H
Note: “straight”Hchain
shown in structural methyl group:
C
H
formula isn’t
CH
Propane: C3H8 or
CH3CH2CH3
H
As represented by
skeletal formula
3
Straight-Chain Alkanes
n = 3 propane
Propane: Molecular formula: C3H8
H
H
Structural formula:
H
H
C
C
C
H
H
H
H
For n > 3, it makes a difference which
carbon the next methyl group is added
For straight-chain alkanes, next methyl
always added to an end carbon – structural
formula (untwisted) has all carbons in a line
Simple Hydrocarbons - Alkanes
Condensed formula helps to see structure
Butane C4 shown as straight-chain isomer
Molecular
Structural
Type of Formula
Ball-and-Stick Space Fill
Condensed
CH3CH3
CH3CH2CH3
CH3CH2CH2CH3
Hydrocarbons – Straight-Chain Alkanes
Name
Type of Formula
Molecular
Condensed
Chapt 21 Hydrocarbons [Selected]
21.1 Introduction to Hydrocarbons
21.2 Alkanes [Straight-Chain Only]
21.3 Alkenes & Alkynes
21.4 Hydrocarbon Isomers [also with O]
Section 21.3 Alkenes & Alkynes
Alkenes are hydrocarbons that contain at
least one double-bond; Alkynes are
hydrocarbons that contain at least one
triple-bond.
• Name a straight-chain alkene or alkyne from its molecular
formula or by examining its structure (up to oct-).
• Draw the structural formula or write the molecular formula
of a straight-chain alkene or alkyne when given its name (up
to oct-).
Alkenes
• Alkenes- hydrocarbons that have
one or more double bonds
between Carbons
• Unsaturated hydrocarbons
• Naming: prefix + ene
Naming and Drawing Alkenes
Prefix
Name
Structure
Eth-
Ethene
H-CH=CH-H
Prop-
Propene
H-CH=CH-CH2-H
But-
1-Butene
H-CH=CH-CH2-CH2-H
Pent- 1-Pentene
Hex-
1-Hexene
H-CH=CH-CH2-CH2-CH2-H
H-CH=CH-CH2-CH2-CH2-CH2-H
Hept- 1-Heptene H-CH=CH-CH2-CH2-CH2-CH2-CH2-H
Oct-
1-Octene
H-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-H
Doubles Bonds Can Move,
Changes Name of Molecule
Prefix
Name
Structure
But-
1-Butene
H-CH=CH-CH2-CH2-H
But-
2-Butene
H-CH2-CH=CH2-CH2-H
Pent- 1-Pentene
H-CH=CH-CH2-CH2-CH2-H
Pent- 2-Pentene
H-CH2-CH=CH-CH2-CH2-H
Hex- 1-Hexene H-CH=CH-CH2-CH2-CH2-CH2-H
Hex- 2-Hexene
H-CH2-CH=CH2-CH2-CH2-CH2-H
Hex- 3-Hexene H-CH2-CH2-CH=CH-CH2-CH2-H
Alkynes
• Alkynes- are hydrocarbons that
have one or more triple bonds
between Carbons
• Unsaturated hydrocarbons
• Naming: prefix + yne
• Example: Ethyne (Acetylene) used
as a fuel in welding.
Naming and Drawing Alkynes
Prefix
Name
Structure
Eth-
Ethyne
H-C=C-H
Prop-
Propyne
H-C=C-CH2-H
But-
1-Butyne
H-C=C-CH2-CH2-H
Pent- 1-Pentyne
Hex-
1-Hexyne
Hept- 1-Heptyne
Oct-
1-Octyne
H-C=C-CH2-CH2-CH2-H
H-C=C-CH2-CH2-CH2-CH2-H
H-C=C-CH2-CH2-CH2-CH2-CH2-H
H-C=C-CH2-CH2-CH2-CH2-CH2-CH2-H
Chapt 21 Hydrocarbons [Selected]
21.1 Introduction to Hydrocarbons
21.2 Alkanes [Straight-Chain Only]
21.3 Alkenes & Alkynes
21.4 Hydrocarbon Isomers [also with O]
Section 21.4 Hydrocarbon Isomers
Some hydrocarbons [and other compounds]
have the same molecular formula but have
different molecular structures.
• Define the terms isomer, structural isomer, and stereoisomer.
• Categorize molecular structures as being structural isomers,
stereoisomers or as not being isomers.
• Distinguish between geometric (diastereomers) and optical
isomers (enantiomers)
• Differentiate between geometric isomers with cis- and trans
prefixes.
• Describe describe the structural characteristics that are
associated with optical isomers
Section 21.4 Hydrocarbon Isomers
Some hydrocarbons [and other compounds]
have the same molecular formula but have
different molecular structures.
• Generate isomers of compounds containing oxygen in
addition to carbon and hydrogen
Section 21.4 Hydrocarbon Isomers
Key Concepts
• Isomers are two or more compounds with the same
molecular formula but different molecular structures.
• Structural isomers differ in the order in which atoms are
bonded to each other.
• Stereoisomers have all atoms bonded in the same order but
arranged differently in space; stereoisomers can either be
optical isomers (enantiomers) or not (diastereomers)
• Stereoisomers which are non-superimposable mirror
images of each other are called optical isomers
• Some diastereomers are geometric isomers; these are
associated with carbon compounds containing double
bonds
Section 21.4 Hydrocarbon Isomers
Key Concepts
• When oxygen is present in a compound with carbon and
hydrogen, isomers can involve hydroxy (-OH), ether (-O-)
and carbonyl (C=O) groupings
Formulas for Compounds - Isomers
Isomers – different compounds which have the
same chemical formula
2 main categories: structural (aka constitutional)
isomers and stereoisomers (aka configurational)
Structural isomer - atoms bonded in different
order
Stereoisomer – atoms bonded in same order but
differ in spatial orientation
Web resources at:
http://www.chemguide.co.uk/basicorg/isomermenu.html#top
http://www.brightstorm.com/science/chemistry/organic-chemistry/isomers-stereoisomers/
Types of Isomers
All Isomers
Structural
(Constitutional)
Stereoisomers
(Configurational)
Diastereomers
Geometric
(Cis-trans)
Enantiomers
(optical)
Other diastereomers
(>1 chiral centers)
Butane – Structural Isomers
Butane, C4H10 - smallest alkane to have
isomers (has two)
Unlike construction of ethane and propane,
choice of which H in propane to replace with
a methyl group makes a difference
Two possible choices generate two
structural isomers – carbons connected to
each other differently
Structural isomers differ in physical and
chemical properties
Making Butane (C4H10) From Propane
or
H H H
H C C C H
H H H
propane (C3H8)
H
C H
H
H H H H
H C C C C H
H H H H
H
H C H
H
H
H C C C H
H H H
Two Equivalent n-Butane Molecules
Right hand structure is twisted
version of left hand molecule
Butane - Bond Rotations (not isomers)
Butane C4H10.
=
Not isomers – carbons connected in same
way and forms can convert from one to the
other by rotating around a bond
=
Structural isomers – carbons connected in
different way; bond must be broken to
convert one form into the other
HStructural
H H HIsomers of C H
H
H H
H4 H10 H H
Isobutane, BP = -12°C
Butane, BP = 0°C
C C
H
H
C
C
C
H H H H
H
H
H C C C C H
H H H H
H H H
H C
H
C
H
C H
H
H
H
HC
H C H
H
H
Skeletal
Formula
n-butane
straight chain
iso-butane
branched
Structural
Formula
H H H H
H C C C C H
H H H H
Other
CH3CH2CH2CH3
Butane (C4H10)
H
H C H
H
H
H C C C H
H H H
CH3CH(CH3)CH3
n-Butane CH3CH2CH2CH3
H H H H
H C C C C H
H H H H
H HH H
H
H
HH H H
Making Pentane (C5H12) From Butane
H H H H
H C C C C H
H H H H
At first glance, appear
H
to be 4 isomers - but
H C H
H
H
H C C C H
H H H
3 isomers remain
The 3 Structural Isomers of Pentane
H
H
H
H
H
H
H
H C
H
H
H
C
C
C
C
H
H
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
H
C
H
H
H
H
H
H
H
Structural Isomers of C5H12 (Pentane)
n-pentane
n
isopentane
neopentane
Longest continuous carbon chain:
pentane 5 isopentane 4 neopentane 3
Alkane Isomers – Alternate Strategy
Rather than building new isomers by adding
a methyl group to known isomers of a
particular alkane, the following slides
illustrate an alternative approach
1) Draw straight chain version of alkane of
interest
2) Break one or more carbon-carbon
bonds and rearrange the pieces
3) Check that new molecules don’t repeat
existing ones (reflection, rotation)
Structural Isomers of Hexane (C6H14)
Start: connect carbons in a line
Break bonds & rearrange to get other isomers
C
C
C
C
C
C
C C C C C
C C C C CC
C
C
C
C
C
C
C
C
Hexane (C6H14) Isomers Showing H Atoms
H
H
H
H
H
H C
C
C
C
C H
C C
H
H
H
H
H
C
H C
H C
H
C
C H
C C
H
H
H
H
H C
C
C C C
C H
H H H H H
H C
H
H
H H H
H
H C
H C
H CC C
C
H C
H C
H
H
H
H
H
H
H
H
H
H C
H C
H
H
H
H
H
C C
C H
H
H
H
H
H
C CC C C CC C
C C
H
H
H C H H C H
H
H
H
Hexane (C6H14) Isomers In Skeleton Form
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
H H H H H
H C
C
H
H C
H
H
C
C
H
H
H H H
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
C
H
H
C
C
H
H
H
C
C
C
H
C
H
H
H
H
H
H
H
H
H
H
C
H
H
H
H
H
H
H
C
C
C
C
H
H
H C H H C H
H
H
H
Structural Isomers of Hexane (C6H14)
1
2
3
1
4
5
2
3
1
2
3
4
4
5
5
Boiling Points - Hexane Isomers
Structural Isomers of Hexane (C6H14)
Site has rotatable models with display options
http://www.creative-chemistry.org.uk/molecules/hexane.htm
# of Alkane (CnH2n+2) Structural Isomers
Molecular Possible # Molecular
Formula
Isomers
Formula
C11H24
C4H10
2
Possible # Isomers
159
C5H12
3
C12H32
355
C6H14
5
C15H32
4,347
C7H16
9
C20H42
366,319
C8H18
18
C30H62
4,111,846,763
C9H20
35
C40H82
62,481,801,147,341
C10H22
75
Types of Isomers
All Isomers
Structural
(Constitutional)
Stereoisomers
(Configurational)
Diastereomers
Geometric
(Cis-trans)
Enantiomers
(optical)
Other diastereomers
(>1 chiral centers)
Enantiomers - Optical Isomers
Tied to concept of non superimposable
mirror images
Familiar example – hands: left & right hands
are mirror images but do not superimpose
Superimposable Mirror Images
Some molecules are like
socks - two socks from
pair are mirror images
that are superimposable
(sock and its mirror image
are identical)
Molecule/object that is
superimposable on its
mirror image is achiral; it
is chiral if it is not
superimposable on its
mirror image
63
Superimposable Mirror Images
All molecules have a mirror image – but for many
molecules it is the same molecule (achiral)
H
H
H
C
C
H
F
H
F
fluoromethane
H
Achiral Examples
65
Nonsuperimposable Mirror Images
Mirror image cannot be rotated so all its atoms
align with same atoms of original molecule – i.e.,
mirror image is not superimposable on original
Non-Superimposable Mirror Images
Only when C attached to 4 different groups
When this occurs, C variously referred to as
a chiral center, asymmetric carbon,
stereogenic center, or stereocenter
Identifying Chiral Centers
Examine each tetrahedral carbon atom and look
at four groups (not the four atoms) bonded to it
If groups all different, have a chiral (stereogenic)
center
Chiral Molecules - Entantiomers
A pair of nonsuperimposable mirror images
are called a pair of enantiomers – these
molecules will be optical isomers of each
other
Chirality – Optical Isomers
Louis Pasteur discovered 2 forms of
crystallized tartaric acid; forms were mirror
images of each other called right and lefthanded forms
Non-Superimposable Mirror Images
Chiral molecule – mirror images are
enantiomers (optical isomers)
Non-Superimposable Mirror Images
Chiral molecule – mirror images are
enantiomers (optical isomers)
H
OH
OH
C
C
COOH
CH3
(-) lactic acid
in sour milk
HOOC
H3C
(+) lactic acid
in muscles
H
Non-Superimposable Mirror Images
CH3
CH
Cl
H
CH2
CH3
2-chlorobutane
CH2CH3
CH2CH3
C
C
CH3
Cl
H3C
Cl
H
Non-Superimposable Mirror Images
OH
CH3
CH3
CH2
CH
butan-2-ol
CH3
CH2CH3
CH2CH3
C
C
H
OH
H
HO
CH3
Return to Heptane – Optical Isomers
Earlier in presentation, structural isomers of
heptane (C7H14) were determined
Some of these isomers are optically active
(following slide)
Structural Isomers of C7H16 (Heptane)
*
*
Isomers marked with * have asymmetric carbons
(have enantiomers – optical isomers)
Optical Isomers of 3-methylhexane
Tro, Chemistry: A Molecular Approach
77
Properties of Optical Isomers
Optical isomers have the same physical*
and chemical properties except in chemical
reactions where chirality is important
Chiral molecules often react differently with
other chiral molecules; similar to idea that
right hand does not fit a left handed glove –
molecule must be correct shape to fit
molecule it is reacting with
* Except for their rotation of polarized light
Impact of Chirality
Many natural molecules are chiral and most
natural reactions are affected by optical
isomerism; e.g. D- & L-amino acids!
Many drugs are optically active, with only
one enantiomer being beneficial
(or harmful, e.g. thalidomide
Types of Isomers
All Isomers
Structural
(Constitutional)
Stereoisomers
(Configurational)
Diastereomers
Geometric
(Cis-trans)
Enantiomers
(optical)
Other diastereomers
(>1 chiral centers)
Types of Isomers
All Isomers
Structural
(Constitutional)
Stereoisomers
(Configurational)
Diastereomers
Geometric
(Cis-trans)
Enantiomers
(optical)
Other diastereomers
(>1 chiral centers)
Stereoisomers – No Chiral Centers
Single covalent bonds can easily rotate - what
appears to be a different structure is not
All structures above are the same (not isomers)
because C-C bonds have free rotation
Stereoisomers – No Chiral Centers
A double bond between carbon atoms prevents
free rotation – structure is locked into place
Stereoisomers – No Chiral Centers
Groups on same or opposite sides of double
bond – diastereomers (spatially different but not
mirror images)
Geometrical Isomers
When double bonds involved, diastereomers
are referred to as geometrical isomers
Although cis and trans modifiers to names are
traditional, official naming system uses E and
Z modifiers
Isomers with CHO Compounds
With carbon, hydrogen and oxygen (CHO)
present, isomers become more varied
Rule: O in these compounds forms 2 bonds
O found in form of alcohol (COH), ether
(COC), or carbonyl (C=O) group
Isomers of C3H8O
2 structural isomers in form of alcohol
1 structural isomer in form of ether
propan-1-ol
propan-2-ol
ethyl methyl ether
Isomers of C4H10O
4 structural isomers in form of alcohol
butan-1-ol
*
butan-2-ol
2-methylpropan-1-ol
* chiral
center
2-methylpropan-2-ol
Isomers of C4H10O
3 structural isomers in form of ether
1-methoxypropane
diethyl ether
2-methoxypropane
Isomers of C4H10O
1 chiral center  2 optical isomers
*
(2S)-butan-2-ol
butan-2-ol
(2R)-butan-2-ol
Isomers of
C4H10O
8 total isomers
Summary – Isomers of C4H10O
4 structural isomers in form of alcohol
1 alcohol has a chiral center  2
enantiomers (optical isomers)
5 total isomers in form of alcohol
3 structural isomers in form of ether
0 chiral centers
3 total isomers in form of ether
8 total isomers (7 structural)
# of CnH2n+2O Structural Isomers
http://www.docbrown.info/page07/isomerism1.htm
Molecular Possible # Molecular
Formula
Isomers
Formula
2
C2H6O
C9H20O
Possible # Isomers
C3H8O
3
C10H22O
989
C4H10O
7
C11H24O
~2430
C5H12O
14
C12H32O
~6070
C6H14O
32
C7H16O
72
C8H18O
171
405
Summary – Isomers
Structural Isomers