Chapt 21 Hydrocarbons [Selected] 21.1 Introduction to Hydrocarbons 21.2 Alkanes [Straight-Chain Only] 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 carbon Hydrocarbons are organic substances composed of carbon and hydrogen. 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 C 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 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)

Bond to some unspecified atom 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 one double or triple bond Bond to some unspecified atom Single Double Triple

Simplest Hydrocarbon - Methane Chemical (molecular) formula: CH4 Structural formula: C H Chemical bond Carbon atom with 4 bonds

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 Chemical (molecular) formula CH4 C H Structural 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 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 possible hydrocarbon = methane Chemical (molecular) formula: CH4 Structural formula: C H Chemical bond Carbon atom with 4 bonds

Alkanes All have formula CnH2n+2 n = integer n = 2 ethane Chemical (molecular) formula: C2H6 Structural formula: n = 3 propane Chemical (molecular) formula: C3H8 C H C 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 methyl group: CH3 ethane C2H6

Ethane Can write formula as C2H6 or as CH3CH3 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 22

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 Note: “straight” chain shown in structural formula isn’t methyl group: CH3 As represented by skeletal formula Propane: C3H8 or CH3CH2CH3

Straight-Chain Alkanes n = 3 propane Propane: Molecular formula: C3H8 Structural formula: 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 C H

Simple Hydrocarbons - Alkanes Condensed formula helps to see structure Butane C4 shown as straight-chain isomer Type of Formula Molecular Structural Ball-and-Stick Space Fill Condensed CH3CH3 CH3CH2CH3 CH3CH2CH2CH3

Hydrocarbons – Straight-Chain Alkanes Type of Formula Name Molecular Condensed

Chapt 21 Hydrocarbons [Selected] 21.1 Introduction to Hydrocarbons 21.2 Alkanes [Straight-Chain Only] 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 # of isomers increase as # of atoms increase Good web resources at: http://www.chemguide.co.uk/basicorg/isomermenu.html#top http://www.brightstorm.com/science/chemistry/organic-chemistry/isomers-stereoisomers/

Structural (Constitutional) Types of Isomers All Isomers Structural (Constitutional) Stereoisomers (Configurational) Diastereomers Enantiomers (optical) Geometric (Cis-trans) 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 propane (C3H8)

Two Equivalent n-Butane Molecules Right hand structure is twisted version of left hand molecule 37

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 40

Butane (C4H10) Skeletal Formula Structural Formula Other n-butane straight chain CH3CH2CH2CH3 Butane (C4H10) iso-butane branched CH3CH(CH3)CH3

n-Butane CH3CH2CH2CH3

Making Pentane (C5H12) From Butane At first glance, appear to be 4 isomers - but 3 isomers remain

The 3 Structural Isomers of Pentane

Structural Isomers of C5H12 (Pentane) n-pentane 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 Draw straight chain version of alkane of interest Break one or more carbon-carbon bonds and rearrange the pieces 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 47

Hexane (C6H14) Isomers Showing H Atoms 48

Hexane (C6H14) Isomers In Skeleton Form 49

Structural Isomers of Hexane (C6H14) 2 3 1 4 5 2 3 1 2 3 5 4 4 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 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

Structural (Constitutional) Types of Isomers All Isomers Structural (Constitutional) Stereoisomers (Configurational) Diastereomers Enantiomers (optical) Geometric (Cis-trans) 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

Superimposable Mirror Images All molecules have a mirror image – but for many molecules it is the same molecule (achiral) 57

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 59

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 60

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 62

Chirality – Optical Isomers Louis Pasteur discovered 2 forms of crystallized tartaric acid; forms were mirror images of each other called right and left-handed 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) 65

Non-Superimposable Mirror Images 2-chlorobutane

Non-Superimposable Mirror Images butan-2-ol

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 70 70

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; most amino acids are chiral Many drugs are optically active, with only one enantiomer having the beneficial effect For some drugs, other enantiomer can even be harmful, e.g. thalidomide

Structural (Constitutional) Types of Isomers All Isomers Structural (Constitutional) Stereoisomers (Configurational) Diastereomers Enantiomers (optical) Geometric (Cis-trans) Other diastereomers (>1 chiral centers)

Structural (Constitutional) Types of Isomers All Isomers Structural (Constitutional) Stereoisomers (Configurational) Diastereomers Enantiomers (optical) Geometric (Cis-trans) 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 can 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 For simplicity, limit scope to single oxygen O found in form of alcohol (COH), ether (COC), or carbonyl (C=O) group Look at: C3H8O, C4H10O, C5H12O, C3H6O Latter departs from other examples – rings and double bonds present

Isomers of C3H8O 2 structural isomers in form of alcohol 1 structural isomer in form of ether ethyl methyl ether propan-1-ol propan-2-ol

Isomers of C4H10O * 4 structural isomers in form of alcohol butan-1-ol 2-methylpropan-1-ol butan-2-ol 2-methylpropan-2-ol * * chiral center

Isomers of C4H10O 3 structural isomers in form of ether 1-methoxypropane diethyl ether 2-methoxypropane

1 chiral center  2 optical isomers 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 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

Summary – Isomers Structural Isomers