Unit 2 – Alkanes and Chemical Reactions Structure and Stereochemistry of Alkanes Nomenclature of alkanes and cycloalkanes Physical Properties Conformational Analysis The Study of Chemical Reactions Kinetics and Thermodynamic Quantities Free Radical Halogenation Reactive Intermediates and Transition States

Hydrocarbons The simplest organic compounds are the hydrocarbons: organic compounds that contain only carbon and hydrogen four general types: alkanes alkenes alkynes aromatic hydrocarbons

Hydrocarbons Alkanes are often called saturated hydrocarbons they contain the maximum number of hydrogen atoms per carbon atom. Alkenes, alkynes, and aromatic hydrocarbons are called unsaturated hydrocarbons they contain fewer H atoms than an alkane with the same number of carbon atoms

Alkanes You must know the names and formulas for the 10 simplest alkanes: CH4 methane CH3CH3 ethane CH3CH2CH3 propane CH3CH2CH2CH3 butane CH3CH2CH2CH2CH3 pentane CH3CH2CH2CH2CH2CH3 hexane

Alkanes You must know the names and formulas for the 10 simplest alkanes: CH3CH2CH2CH2CH2CH2CH3 heptane CH3CH2CH2CH2CH2CH2CH2CH3 octane CH3CH2CH2CH2CH2CH2CH2CH2CH3 nonane CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 decane

Alkanes The alkanes form a homologous series with a general molecular formula of CnH(2n+2) Homologous series: a series of compounds in which each member differs from the next member by a constant unit Alkanes differ from each other by -CH2-

Alkanes Example: Which of the following are alkanes: C2H6, C3H6, C5H12, C4H8 Example: What is the formula for an alkane with 12 carbons?

Alkanes The previous alkanes are straight-chain alkanes: all of the carbon atoms are joined in a continuous chain also called “normal” alkanes (n-alkanes) Alkanes containing 4 or more carbons can also form branched alkanes one or more of the carbon atoms form a “branch” or side-chain off of the main chain

Alkanes 2-methylbutane 2,2-dimethylpropane An example of a straight chain alkane: C5H12 pentane Examples of branched alkanes: C5H12 2-methylbutane 2,2-dimethylpropane

Alkanes The three structures shown previously for C5H12 are structural (constitutional) isomers: compounds with the same molecular formula but different bonding arrangements Structural isomers have different properties: different melting points different boiling points often different chemical reactivity

Alkane Nomenclature pentane 2-methylbutane or isopentane Organic compounds can be named either using common names or IUPAC names. pentane 2-methylbutane or isopentane 2,2-dimethylpropane or neopentane

Alkane Nomenclature “iso” The common name for any alkane containing a CH3 group on the second carbon in the chain is “isoalkane.” “iso” Isobutane (4 C total) Isohexane (6 C total)

Alkane Nomenclature Most of the time, organic chemists use the IUPAC names for organic compounds. LEARN THE RULES FOR EACH CLASS OF COMPOUNDS WE DISCUSS.

Alkane Nomenclature Base name: heptane To name an alkane: Find the longest continuous chain of carbon atoms and use the name of that chain as the base name of the compound: the longest chain is often NOT written in a straight line Base name: heptane

Alkane Nomenclature A substituent Number the carbon atoms in the longest chain starting at the end of the chain closest to a substituent a group attached to the main chain that has taken the place of a hydrogen atom on the main chain A substituent

Alkane Nomenclature Name and give the location of each substituent. Common substituents: Halo group a halogen atom “Halo” groups are named using “halo”: Cl chloro Br bromo I iodo F fluoro Nitro group -NO2

Alkane Nomenclature Common substituents: alkyl group A group that is formed by removing an H atom from an alkane the alkyl group attaches to the main chain at the carbon that has lost its H

Alkane Nomenclature Alkyl groups are named by replacing the “ane” ending of the parent alkane with the “yl” ending.

Alkane Nomenclature

Alkane Nomenclature

Alkane Nomenclature Complex alkyl substituents Use the longest alkyl chain of the substituent as the base name of the substituent Number the substituent chain with the “head carbon” as carbon 1 List substituents on the alkyl chain with the appropriate numbers Use parentheses around the name of the group

Alkane Nomenclature Methyl group 3-methylheptane Note: Separate numbers from letters using a hyphen. Separate groups of numbers using commas.

Alkanes Know these. Note: Ignore these prefixes when alphabetizing. Alkane Nomenclature: When two or more substituents are present, list them in alphabetical order: isopropyl before methyl t-butyl or sec-butyl before chloro When more than one of the same substituent is present (i.e. two methyl groups), use prefixes to indicate how many. Give the location of each as well. Di = two Tri = three Tetra = four Penta = five Hexa = six Know these. Note: Ignore these prefixes when alphabetizing.

Alkane Nomenclature Additional rules: When there are two “longest” chains of equal length, use the chain with the greater number of substituents. correct incorrect

Alkane Nomenclature Additional rules: If each end of the longest chain has a substituent the same distance from the end, start with the end nearer to the second substituent. correct incorrect 3-chloro-2,5-dimethylhexane

Alkanes Example: Name the following compounds:

Alkanes Example: Name the following compound:

Alkanes You must also be able to write the structure of an alkane when given the IUPAC name. Identify the main chain and draw the carbons in it. Identify the substituents (type and #) and attach them to the appropriate carbon atoms on the main chain. Add hydrogen atoms to the carbons to make a total of 4 bonds to each carbon

Alkanes Example: Draw the structure for the following compounds: 3, 3-dimethylpentane 4-sec-butyl-2-methyloctane 1,2-dichloro-3-methylheptane 2-nitropropane

Alkane Nomenclature Example: Draw the structural isomers of hexane (C6H14). Name each isomer. Use a systematic approach to draw structural isomers: Draw the unbranched isomer for the first structure. For other structures, remove 1 or more carbons (and/or functional groups) from the unbranched isomer and reposition to make unique compounds

Types of Carbon Atoms Primary carbon (1o) a carbon bonded to one other carbon Secondary carbon (2o) two other carbons Tertiary carbon (3o) three other carbons

Physical Properties Solubility Alkanes are nonpolar hydrophobic do not dissolve in water soluble in nonpolar or weakly polar organic solvents Density: varies from ~0.5 - ~0.8 g/mL less dense than water (1.0 g/mL) Alkanes float on water

Physical Properties Boiling Point In general, boiling point increases as the molecular weight of the alkane increases larger molecules have greater surface area and higher London dispersion forces of attraction must be overcome for vaporization and boiling to occur

BP (branched) < BP (n-alkane) Physical Properties Boiling Point (cont) Given the same total number of carbon atoms: BP (branched) < BP (n-alkane) Branched alkanes are more compact. less surface area smaller London dispersion forces lower BP

Physical Properties Melting Points: In general, melting point increases as MW increases irregular, sawtooth pattern

Physical Properties Melting Point: Alkanes with odd number of carbons have lower than expected melting points (compared to the previous alkane with an even number of carbons) Even # carbons better packing in solid structure higher MP Odd # carbons do not pack as well lower MP

MP (branched) > MP (n-alkane) Physical Properties Melting Points: Given the same total number of carbon atoms: MP (branched) > MP (n-alkane) branched alkanes have more compact structure better packing higher MP

Sources & Uses of Alkanes Alkanes are derived primarily from petroleum and petroleum by-products: Refining via fractional distillation gives useful mixtures of alkanes: C2 - C4 liquified petroleum gas C4 - C9 gasoline C8 - C16 kerosene C10 - C18 diesel C16+ heavy/mineral oil

alkane smaller alkanes + alkenes Reactions of Alkanes Catalytic Cracking: converts alkanes into more valuable mixtures of smaller alkanes and alkenes alkane smaller alkanes + alkenes C12H26 D SiO2 or Al2O3 catalyst D SiO2 +

Alkane shorter alkanes Reactions of Alkanes Hydrocracking: converts higher alkanes into shorter alkanes and eliminates N and S impurities Alkane shorter alkanes C12H26 D H2, SiO2 or Al2O3 catalyst D + H2, SiO2

Reactions of Alkanes Combustion: a rapid, exothermic redox reaction that converts hydrocarbons into carbon dioxide and water alkane + O2 CO2 + H2O 2 C6H14 + 19 O2 12 CO2 + 14 H2O (unbalanced)

alkane + X2 mixture of alkyl halides Reactions of Alkanes Halogenation: a reaction in which a halogen atom is substituted for a hydrogen atom on an alkane alkane + X2 mixture of alkyl halides CH4 + Cl2 CH3Cl + CH2Cl2 + CHCl3 + CCl4 D or hu hu unbalanced

Conformations of Alkanes The simplest alkane, CH4, is perfectly tetrahedral: bond angle = 109.5 C-H bond length = 1.09 A free rotation of the C-H bond Figure: 03-04-08UN Caption: The simplest alkane is methane, CH4. Methane is perfectly tetrahedral, with the 109.5° bond angles predicted for an sp3 hybrid carbon. The four hydrogen atoms are covalently bonded to the central carbon atom, with bond lengths of 1.09 Å.

Conformations of Alkanes Ethane: Two carbons overlapping sp3 hybrid orbitals form a sigma bond Figure: 03-04-09UN Caption: Ethane, the two-carbon alkane, is composed of two methyl groups with overlapping sp3 hybrid orbitals forming a sigma bond between them.

Conformations of Alkanes The two methyl groups are relatively free to rotate about the sigma bond between the two carbon atoms sigma bond maintains its overlap at all times The different arrangements formed by rotation around a single bond are called conformations. Conformer: a specific conformation a “conformational isomer”

Conformations of Alkanes Conformers are often drawn using Newman projections: a way of drawing a molecule looking straight down the bond connecting two carbon atoms front carbon atom is represented by three lines joined together in a Y shape back carbon is represented by a circle with three bonds pointing out from it

Conformations of Alkanes View from this end = 3-D structure of one conformer of ethane Newman projection

Conformations of Alkanes An infinite number of conformations are possible for ethane and higher alkanes. The dihedral angle (q) can have an infinite number of values angle between the C-H bonds on the front and back carbons q

Conformations of Alkanes Important conformations for ethane: Eclipsed conformation Staggered conformation Figure: 03-06 Caption: Ethane conformations. The eclipsed conformation has a dihedral angle  = 0, and the staggered conformation has q = 60. Any other conformation is called a skew conformation. Skew conformation Molecules are constantly rotating through all possible conformations.

Conformations of Alkanes The conformation of ethane changes constantly at room temperature. Conformations may have different energies. Lowest energy conformer is most favored. Highest energy conformer is least favored. Conformational analysis: the study of the energies of different conformations helps predict which conformation are favored and which reaction may occur

Conformations of Alkanes Staggered conformation of ethane: lowest energy most favored electron clouds in the C - H bonds separated as much as possible Eclipsed conformation of ethane: highest energy least favored electron clouds of C - H bonds are closest together

Conformations of Alkanes As ethane rotates from the staggered conformation towards the eclipsed conformation: potential energy increases due to torsional strain resistance to rotation or twisting about a bond

Conformations of Alkanes Figure: 03-07 Caption: The torsional energy of ethane is lowest in the staggered conformation. The eclipsed conformation is about 3.0 kcal/mol (12.6 kJ/mol) higher in energy. At room temperature, this barrier is easily overcome, and the molecules rotate constantly.

Conformation of Alkanes Butane: 4 carbon chain held together by end-to-end overlap of sp3 hybrid orbitals on the carbon atoms tetrahedral geometry around each carbon free rotation about any C - C bond many conformers of differing energies are possible Newman projections of butane are drawn by looking down the central C2 - C3 bond.

Conformations of Alkanes Figure: 03-10 Caption: Butane conformations. Rotations about the center bond in butane give different molecular shapes. Three of these conformations are given specific names. Totally eclipsed (0o) Gauche (60o) Eclipsed (120o) Anti (180o)

Conformation of Alkanes Totally eclipsed conformer of butane: highest energy due to steric hinderance between the methyl groups Steric hinderance: interference between two bulky groups that are close enough together so that their electron clouds repel each other

Conformations of Alkanes Figure: 03-11-01UN Caption: The totally eclipsed conformation is about 1.4 kcal (5.9 kJ) higher in energy than the other eclipsed conformations, because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion. This kind of interference between two bulky groups is called steric strain or steric hindrance.

Conformations of Alkanes Eclipsed conformer of butane: second highest in energy due to repulsion of the methyl group on one carbon and the hydrogen atom on the other All staggered conformers (gauche and anti) of butane are lower in energy than any of the eclipsed conformers. Anti conformer of butane: lowest energy because methyl groups are furthest apart

Conformations of Alkanes Figure: 03-11 Caption: Torsional energy of butane. The anti conformation is lowest in energy, and the totally eclipsed conformation is highest in energy.