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