chapt 21 hydrocarbons [selected]
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Chapt 21 Hydrocarbons [Selected]21.1 Introduction to Hydrocarbons21.2 Alkanes [Straight-Chain Only]21.4 Hydrocarbon Isomers [also with O]
Section 21.1 Introduction to Hydrocarbons
• 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.
Hydrocarbons are carbon-containing organic compounds that provide a source of energy and raw materials.
Section 21.1 Introduction to Hydrocarbons
Key Concepts
• Organic compounds contain the element carbon
• Hydrocarbons are organic substances composed of carbon and hydrogen.
• The major sources of hydrocarbons are petroleum and natural gas.
Organic CompoundsName used because living organisms known to contain/produce themTerm applied to all carbon-containing compounds except for a small number of compounds considered to be inorganicBecause huge number exist, an entire branch of chemistry – organic chemistry – devoted to their study
Essential Organic ChemistryBond – force that hold atoms together in compoundsCarbon atom (C) always forms 4 bonds with other atoms; bond represented by a lineC forms covalent bonds (a particular type of bond)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 hydrogenAre thousands of hydrocarbons; can be in chain, branched chain, ring, and cage-like structuresMajor source of hydrocarbons – petroleum and natural gas (mostly CH4 – methane)
HydrocarbonsCarbon atoms bond to each other by single, double, & triple bonds (always 4 total bonds)Saturated hydrocarbons contain only single bondsUnsaturated hydrocarbons contain at least one double or triple bond
Single Double Triple
Bond to some unspecified
atom
Simplest Hydrocarbon - MethaneChemical (molecular) formula: CH4
Structural formula:
C
H
H
H
HChemical bond
Carbon atom with 4 bonds
Simple Hydrocarbons - MethaneOne carbon atom attached to 4 hydrogensShape of carbon bonded to 4 other atoms is a tetrahedron – bond angles of 109.5Hydrogens occupy corners of tetrahedron
109.5
Ways of Representing CompoundsCompounds may be represented by various types of formulas and graphical presentationsVariety of these shown on following slidesChemists use form that best shows information they wish to highlightMolecular formula most compact but no information about connections & geometryStructural show connections but no 3D infoMost graphical forms can be generated and/or manipulated using online chemical software
Ways of Representing MethaneChemical (molecular) formula CH4
C
H
H
H
HStructural formula
Structural formula with additional geometry information – solid wedge coming toward you, dashed one away
Ball & stick
model
Space-filling model
Chapt 21 Hydrocarbons [Selected]21.1 Introduction to Hydrocarbons21.2 Alkanes21.4 Hydrocarbon Isomers [also with O]
Section 21.2 Alkanes
• 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).
Alkanes are hydrocarbons that contain only single bonds.
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.
AlkanesSimple Alkanes – hydrocarbons with only single bonds and no ring structuresAll have formula CnH2n+2 n = integer
All have names ending in “ane”Simplest possible hydrocarbon = methaneChemical (molecular) formula: CH4
Structural formula: C
H
H
H
HChemical bond
Carbon atom with 4 bonds
AlkanesAll have formula CnH2n+2 n = integern = 2 ethaneChemical (molecular) formula: C2H6
Structural formula:
n = 3 propaneChemical (molecular) formula: C3H8
Structural formula:
C
H
H
H
C
H
H
H
C
H
H
HC
H
H
H
C
H
H
“Constructing” Alkanes StepwiseCan 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
methaneCH4
H CH
HH
Making Ethane (C2H6) From CH4
C HH
H
methyl group: CH3
ethaneC2H6
Ethane
Can write formula as C2H6 or as CH3CH3
Latter method allows one to visualize and draw structure more easilyAdditional 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 meetH not shown unless attached to drawn atom
As represented by skeletal formula
Ethane
Most alkanes rotate freely about the single bond between carbon atoms
Single Bond Free RotationFree 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 RotationFree rotation occurs about single bonds
Because of free rotation all six hydrogen atoms in ethane are equivalentIf making new compound from ethane by replacing a hydrogen, doesn’t matter which one is chosen – result will be the same
H CH
HC HH
HC HH
H
Making Propane (C3H8) From Ethane
Ethane: C2H6
methyl group: CH3
Propane: C3H8 or CH3CH2CH3
As represented by skeletal formula
Note: “straight” chain shown in structural
formula isn’t
Straight-Chain Alkanesn = 3 propanePropane: Molecular formula: C3H8
Structural formula:
For n > 3, it makes a difference which carbon the next methyl group is addedFor straight-chain alkanes, next methyl always added to an end carbon – structural formula (untwisted) has all carbons in a line
C
H
H
HC
H
H
H
C
H
H
Simple Hydrocarbons - AlkanesCondensed formula helps to see structureButane C4 shown as straight-chain isomer
CH3CH2CH2CH3
Type of FormulaMolecular Structural Ball-and-Stick Space Fill Condensed
CH3CH3
CH3CH2CH3
Hydrocarbons – Straight-Chain AlkanesType of Formula
Name Molecular Condensed
Chapt 21 Hydrocarbons [Selected]21.1 Introduction to Hydrocarbons21.2 Alkanes [Straight-Chain Only]21.4 Hydrocarbon Isomers [also with O]
Section 21.4 Hydrocarbon Isomers
• 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
Some hydrocarbons [and other compounds] have the same molecular formula but have different molecular structures.
Section 21.4 Hydrocarbon Isomers
• Generate isomers of compounds containing oxygen in addition to carbon and hydrogen
Some hydrocarbons [and other compounds] have the same molecular formula but have different molecular structures.
Section 21.4 Hydrocarbon IsomersKey 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 IsomersKey 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 - IsomersIsomers – different compounds which have the same chemical formula2 main categories: structural (aka constitutional) isomers and stereoisomers (aka configurational)Structural isomer - atoms bonded in different orderStereoisomer – atoms bonded in same order but differ in spatial orientation# of isomers increase as # of atoms increaseGood web resources at:http://www.chemguide.co.uk/basicorg/isomermenu.html#tophttp://www.brightstorm.com/science/chemistry/organic-chemistry/isomers-stereoisomers/
Geometric (Cis-trans)
Other diastereomers(>1 chiral centers)
Diastereomers Enantiomers(optical)
Types of Isomers
Structural (Constitutional)
Stereoisomers(Configurational)
All Isomers
Butane – Structural IsomersButane, 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 differenceTwo possible choices generate two structural isomers – carbons connected to each other differentlyStructural isomers differ in physical and chemical properties
propane (C3H8)
H CH
HCH
HC HH
H
C HH
Hor
Making Butane (C4H10) From Propane
H CH
HCH
HC CH
H
H
HH
H CH
HC
C
HC HH
H
H HH
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
Structural Isomers of C4H10
Butane, BP = 0°C Isobutane, BP = -12°C
C C C C
H
H
H H
H H
H H
H
H
C
H
H
H
H H
H
C HC
CH H
H
C
H
C
H
H
H
C
H
H
C
H
H
H
H
C
H
C
H
H
H
C
C
H
H
H
H
H
H
C
H
C
H
H
H
C
H
H
C
H
H
H
H
C
H
C
H
H
H
C
C
H
H
H
H
H
H
Butane (C4H10)
H CH
HCH
HC CH
H
H
HH
H CH
HC
C
HC HH
H
H HH
OtherSkeletal Formula
Structural Formula
CH3CH2CH2CH3
CH3CH(CH3)CH3
n-butane straight chain
iso-butane branched
n-Butane CH3CH2CH2CH3
C CH
HH
H
HCH
HCH
HH
HH
H H
H HH H
HH
H CH
HCH
HC CH
H
H
HH
H CH
HC
C
HC HH
H
H HH
Making Pentane (C5H12) From Butane
At first glance, appear to be 4 isomers - but 3 isomers remain
The 3 Structural Isomers of Pentane
C C C C CH
H H H H H
H
HHHHH
C CH
H H H
HHHH
C HC
H H
H
C
C CH
H H
HH
H
H H
H
C
C
H H
H
C
n
Structural Isomers of C5H12 (Pentane)
n-pentane isopentane neopentane
Longest continuous carbon chain:pentane 5 isopentane 4 neopentane 3
Alkane Isomers – Alternate StrategyRather than building new isomers by adding a methyl group to known isomers of a particular alkane, the following slides illustrate an alternative approach1) Draw straight chain version of alkane of
interest2) Break one or more carbon-carbon
bonds and rearrange the pieces3) Check that new molecules don’t repeat
existing ones (reflection, rotation)
Structural Isomers of Hexane (C6H14)
C C C C C CC C C C C
C
C C C C C
C C C C C
C
C
C C C C
C C
Start: connect carbons in a lineBreak bonds & rearrange to get other isomers
C C C C
C
CH
H
H
H H H
H
H
H
HHHH
H
C C C C
C
C
H
H
H H H
HHHH
H
H
H
H
H
C C C C C CH
H
H
H
H
H
H
H
H
H H
H
HH
C C C C
C
H
H
H H
HHHH
H
HH
C H
H
H
C C C C C C
C C C C C
C
C C C C C
C
C C C C
C
CC C C C
C C
C C C C
C
H
H
H H
HCHH
H
HH
H
HH
H
Hexane (C6H14) Isomers Showing H Atoms
C C C C
C
CH
H
H
H H H
H
H
H
HHHH
H
C C C C
C
C
H
H
H H H
HHHH
H
H
H
H
H
C C C C C CH
H
H
H
H
H
H
H
H
H H
H
HH
C C C C
C
H
H
H H
HHHH
H
HH
C H
H
H
C C C C
C
H
H
H H
HCHH
H
HH
H
HH
H
Hexane (C6H14) Isomers In Skeleton Form
Structural Isomers of Hexane (C6H14)1
1
1
2
2
2
3
4
3
3
4
4
5
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
Molecular Formula
Possible # Isomers
Molecular Formula
Possible # Isomers
C4H10 2 C11H24 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
# of Alkane (CnH2n+2) Structural Isomers
Geometric (Cis-trans)
Other diastereomers(>1 chiral centers)
Diastereomers Enantiomers(optical)
Types of Isomers
Structural (Constitutional)
Stereoisomers(Configurational)
All Isomers
Enantiomers - Optical IsomersTied to concept of non superimposable mirror imagesFamiliar example – hands: left & right hands are mirror images but do not superimpose
56
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
Superimposable Mirror Images
Superimposable Mirror ImagesAll molecules have a mirror image – but for many molecules it is the same molecule (achiral)
fluoromethane
H
CH F
H
H
CHF
H
58
Achiral Examples
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 ImagesOnly when C attached to 4 different groupsWhen this occurs, C variously referred to as a chiral center, asymmetric carbon, stereogenic center, or stereocenter
Examine each tetrahedral carbon atom and look at four groups (not the four atoms) bonded to itIf groups all different, have a chiral (stereogenic) center
Identifying Chiral Centers
Chiral Molecules - EntantiomersA pair of nonsuperimposable mirror images are called a pair of enantiomers – these molecules will be optical isomers of each other
Chirality – Optical IsomersLouis Pasteur discovered 2 forms of crystallized tartaric acid; forms were mirror images of each other called right and left-handed forms
Non-Superimposable Mirror ImagesChiral molecule – mirror images are enantiomers (optical isomers)
Non-Superimposable Mirror ImagesChiral molecule – mirror images are enantiomers (optical isomers)
OH
CH CH3
COOH
OH
CHH3C
HOOC
(-) lactic acid (+) lactic acidin sour milk in muscles
2-chlorobutane
CH3 CH CH2 CH3
Cl
CH2CH3
CH Cl
CH3
CH2CH3
CHCl
H3C
Non-Superimposable Mirror Images
butan-2-ol
CH3 CH2 CH
OH
CH3
CH2CH3
CCH3 OH
H
CH2CH3
CCH3
HHO
Non-Superimposable Mirror Images
Earlier in presentation, structural isomers of heptane (C7H14) were determined
Some of these isomers are optically active (following slide)
Return to Heptane – Optical Isomers
Structural Isomers of C7H16 (Heptane)
Isomers marked with * have asymmetric carbons (have enantiomers – optical isomers)
*
*
Tro, Chemistry: A Molecular Approach 70
Optical Isomers of 3-methylhexane
Optical isomers have the same physical* and chemical properties except in chemical reactions where chirality is importantChiral 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
Properties of Optical Isomers
Many natural molecules are chiral and most natural reactions are affected by optical isomerism; most amino acids are chiralMany drugs are optically active, with only one enantiomer having the beneficial effectFor some drugs, other enantiomer can even be harmful, e.g. thalidomide
Impact of Chirality
Geometric (Cis-trans)
Other diastereomers(>1 chiral centers)
Diastereomers Enantiomers(optical)
Types of Isomers
Structural (Constitutional)
Stereoisomers(Configurational)
All Isomers
Geometric (Cis-trans)
Other diastereomers(>1 chiral centers)
Diastereomers Enantiomers(optical)
Types of Isomers
Structural (Constitutional)
Stereoisomers(Configurational)
All Isomers
Stereoisomers – No Chiral CentersSingle 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 CentersA double bond between carbon atoms prevents free rotation – structure is locked into place
Stereoisomers – No Chiral CentersGroups can on same or opposite sides of double bond – diastereomers (spatially different but not mirror images)
Geometrical IsomersWhen 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 CompoundsWith carbon, hydrogen and oxygen (CHO) present, isomers become more variedRule: O in these compounds forms 2 bondsFor simplicity, limit scope to single oxygenO found in form of alcohol (COH), ether (COC), or carbonyl (C=O) groupLook at: C3H8O, C4H10O, C5H12O, C3H6O
Latter departs from other examples – rings and double bonds present
Isomers of C3H8O2 structural isomers in form of alcohol1 structural isomer in form of ether
propan-1-ol
propan-2-ol
ethyl methyl ether
Isomers of C4H10O4 structural isomers in form of alcohol
butan-1-ol
butan-2-ol
2-methylpropan-1-ol
2-methylpropan-2-ol
* chiral center
*
Isomers of C4H10O3 structural isomers in form of ether
1-methoxypropane
2-methoxypropane
diethyl ether
Isomers of C4H10O1 chiral center 2 optical isomers
(2S)-butan-2-ol
(2R)-butan-2-ol
butan-2-ol
*
Isomers of C4H10O
8 total isomers
Summary – Isomers of C4H10O4 structural isomers in form of alcohol1 alcohol has a chiral center 2 enantiomers (optical isomers)
5 total isomers in form of alcohol3 structural isomers in form of ether0 chiral centers
3 total isomers in form of ether 8 total isomers (7 structural)
Molecular Formula
Possible # Isomers
Molecular Formula
Possible # Isomers
C2H6O 2 C9H20O 405
C3H8O 3 C10H22O 989
C4H10O 7 C11H24O ~2430
C5H12O 14 C12H32O ~6070
C6H14O 32
C7H16O 72
C8H18O 171
# of CnH2n+2O Structural Isomershttp://www.docbrown.info/page07/isomerism1.htm
Structural Isomers
Summary – Isomers
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