Chapter 4- Alkanes Hydrocarbons • Alkanes• Alkenes• Alkynes- • Cycloalkanes- alkanes in which all or some of the carbon atoms are arranged in rings General Formulas • Alkanes- CnH2n+2 • Alkenes- CnH2n • Alkynes- CnH2n-2 • Cycloalkanes- CnH2n Sources • Petroleum • Cracking – Catalytic Cracking- special catalysts are used to break larger alkanes, C12 or more, into smaller ones, C5-10 – Thermal Cracking- same thing except heat is used instead of catalysts Gasoline • 2,2,4-trimethylpentane- “isooctane” • Heptane • Octane Rating Shapes of Alkanes • All carbons are sp3 hybridized and tetrahedral in alkanes and cycloalkanes • Straight Chain- refers to compound being unbranched, not actually straight. They contain only primary and secondary carbons Naming • Development of the formal naming system only came about in the late 1800’s. • Since many organic molecules had already been discovered, the older names are referred to as common names • The formal naming system was created by the International Union of Pure and Applied Chemistry. (IUPAC for short) Names of Straight Chains • Like learning to count in Organic Chemistry • Prefix tells number of carbons, -ane suffix identifies as an Alkane Naming unbranched alkyl groups • Remember, remove a H from an alkyl, you have an alkyl group. • when the H is removed from the end, it is a terminal alkyl group, or Unbranched alkyl group. • To name it through IUPAC, we drop the –ane and add -yl Naming Branched Chains • Rules: 1) Locate the longest continuous chain of carbons 2) Number the longest chain beginning with the end nearest the substituent 3) Use the numbers obtained above to designate the location of every substituent Note: numbers are separated from letters with a hyphen. Naming Branched Chains, cont 4) When 2 or more substituents are present, give each a number corresponding to its location on the longest chain Note: Substituents are listed alphabetically in front of the parent name 5) When two substituents are on the same carbon, use the number twice! Naming Branched Chains, cont 6) When two or more substituents are identical, indicate this by using numerical prefixes such as di-, tri-, tetra-, penta-, etc. Note: numerical prefixes are not included with alphabetizing. Note: make sure each substituent has a number! Multiple numbers are separated by a comma Naming Branched Chains, cont • Most situations can be handled by these 6 fundamental rules • There are two more rules for more complex compounds: Naming Branched Chains, cont 7) When two chains compete for the longest parent chain, choose the chain with the greater number of substituents 8) When branching first occurs at equal distance from either end of the longest chain, choose the chain that gives the lowest number at the first point of difference. Examples Naming Branched Alkyl Groups • Some common names are accepted in IUPAC – Ex. • The prefixes sec- and tert- are NOT included in alphabetizing. • In IUPAC, similar to regular naming except numbering always begins with the Carbon attached to the main chain. Classification of Hydrogens • Covered previously • Same as alcohols and alkyl halides Naming Alkyl Halides • Alkanes with halogen substituents are named in IUPAC as haloalkanes • Cl= Chloro F= Fluoro Br= Bromo I= Iodo • When the chain bears both halo and alkyl substituents, number the chain from the end nearest the 1st substituent regardless if it is a halo or alkyl Naming Alkyl Halides • If the 1st substituent is equal distance from the end, start with the end with the substituent with alphabetical precedence • Some common names still accepted. • In common naming, they are named as alkyl halides Nomenclature of Alcohols • In IUPAC substitutive nomenclature, a name may have four parts: Locants, prefixes, parent name, suffixes • In general, the numbering of the chain always begins at the end nearer the group named as a suffix • The locate for the suffix can come before the parent name or after. Nomenclature of Alcohols • For the rules, we will take our basic rules for alkanes and just adapt them to handle alcohols. • Rules: 1) Select the longest continuous chain of carbons which contains the carbon bonded to the hydroxyl group. Change the parent name by dropping the –e and adding –ol Nomenclature of Alcohols 2) Number the longest continuous chain so as to give the carbon atoms bearing the –OH the lowest number Compounds with 2 –OH groups are named as diol, but you don’t drop the –e. Common Names for Alcohol • Alcohols are named as alkyl alcohol in the common system. Naming Monocyclic Compounds • Monocyclic Compounds- hydrocarbons that only contain 1 ring. • Add the prefix cyclo- to the alkane name having the same number of carbons Substituted Cycloalkance • These are named as: – Alkylcycloalkanes – Halocycloalkanes – Alkylcycloalkanols, etc • If only one substituent is present, no need to number • When two substituents are present, start numbering at the substituent first in alphabetical precedence and continue in the direction to give the next substituent the lowest possible number Substituted Cycloalkance • When three or more are present, we begin at the substituent that gives the lowest set of numbers. • All that being said, group priority still overrules the above! Substituted Cycloalkance • When single ring system is attached to a single chain with a greater number of carbons or when there are more than one ring systems, rings are named as a prefix as cycloalkyls. • We are not covering the naming of Bicyclic Compounds! Naming Alkenes/Cycloalkenes • Rules: 1) Determine the longest continuous chain that contains both carbons of the double bond. Name as the alkane, but drop the –ane and add –ene 2) Number the chain in order to give the carbons of the double bond the lowest numbers the locant for the double bond will be the number of the first carbon of the double bond Naming Alkenes/Cycloalkenes 3) Indicate substituents groups by the number of the carbon they are attached to 4) Number substituted cycloalkenes in the way that gives the carbons in the double bond the 1 and 2 position and that also gives the substituent groups the lower number at the first point of difference Naming Alkenes/Cycloalkenes 5) Name compounds containing both an alcohol and a double bond as alkenols or cycloalkenols and give the carbon bonded to the –OH group the lowest number. 6) Double bonds are sometimes named at substituents. They are named as alkenyls. Naming Alkenes/Cycloalkenes 7) Remember that all double bonds that qualify for cis/trans isomers must be identified by their configuration. The cis/trans designators go at the very beginning of the name. In most double bonds, cis/trans is determined by the orientation of the parent chain. Nomenclature of Alkynes • Same as alkenes, except you drop the –ane and add –yne. • Double bonds have priority over triple bonds Review of Priorities in Nomenclature • Of the groups covered thus far: 1) Alcohols 2) Double bonds 3) Triple bonds 4) Halo/Alkyl substituents More on Alkynes • Monosubstituted Alkynes are called terminal alkynes and the hydrogen attached is called an acetylenic hydrogen. • The anion obtained by removing the acetylenic hydrogen is named as an alkynide ion. Physical Properties of Alkanes and Cycloalkanes • Boiling Point – In general, BP increases as MW increases – Branching lowers BP • Density – As a class, alkanes and cycloalkanes are the least dense of all organic compounds – Density is considerably less than water so they float on top of water Physical Properties of Alkanes and Cycloalkanes • Solubility– Since alkanes and cycloalkanes are essentially completely nonpolar, they are insoluble in water – They do dissolve in each other and other nonpolar solvents. Sigma Bonds and Bond Rotation • Conformations- temporary molecular shapes that result from the rotation of groups about a single bond • Each possible structure is called a conformer • An analysis of the energy changes associated with the molecule undergoing rotation about a single bond is called conformational analysis More structural formulas • Dash formulas • 3D dash formulas • Saw-horse structures • Newman Projections Conformational analysis • The energy difference between conformations can be shown on a potential energy diagram • The difference in energies is called the torsional barrier of a single bond and hinders free rotation. Conformational Analysis of Ethane What does this mean? • Ethane will spend most of the time in the lowest energy, staggered conformation, or close to it. • Occasionally, it will acquire enough energy to overcome the torsional barrier and rotate through the eclipsed conformation. Definitions • Resistance to rotation is collectively called torsional strain. • One component of torsional strain is Steric Hindrance. • Steric Hindrance- An effect on relative reaction rates caused when the spatial arrangement of atoms or groups at or near the reacting site hinders or retards a reaction Conformational Analysis of Butane • The two Gauche conformation are stereoisomer • Since they can be interconverted by the rotation around a bond, they are called Conformational Stereoisomers Relative Stability of Cycloalkanes • Cycloalkanes do not all have the same relative stability • Cyclohexane is the most stable cycloalkane and cyclopropane and cyclobutane are much less stable • The relative instability of cyclopropane and cyclobutane is a direct consequence of their cyclic structure, and are said to posses ring strain. Origin of Ring Strain • The carbon atoms of alkanes and cycloalkanes are sp3 hybridized so they should have bond angles of 109.5o • In cyclopropane, it forms a triangle, so the bond angles inside the ring must be 60o • That is 49.5o smaller than the 109.5o • This compression of the internal bond angle is called Angle Strain. Angle Strain • The sp3 orbitals can not overlap efficiently, therefore causes the carbon-carbon bonds to be weaker • As a result, the molecule has greater potential energy • While the angle strain of cyclopropane accounts for most of the ring strain, it does not account for all of it. Origin of Ring Strain • In addition to angle strain, we have some torsional strain as well because the hydrogens are eclipsed. • So ring strain = angle strain + torsional strain Cyclobutane • Cyclobutane also has considerable angle strain • The internal angles are 88 degrees, a 21o departure from 109.5o • The cyclobutane ring is not planar, it is slightly folded • If cyclobutane were planar, the angle strain would be slightly less with 90o bond angles. Cyclobutane • However, if it were planar, the torsional strain would be much greater because all 8 C-H bonds would be eclipsed • By folding or bending slightly, the ring relieves more torsional strain than it gains in angle strain. Cyclopentane • The internal angles of a regular pentagon is 108o, very close to the 109.5o, of a normal sp3 carbon • Therefore, if cyclopentane were planar, it would have very little angle strain, however, it would have a lot of torsional strain because all 10 C-H bonds would be eclipsed. • As a result, cyclopentane assumes a slightly bent conformation in which one or two of the atoms of the ring are out of the plane Cyclopentane • This relieves some of the torsional strain and allows for slight twisting of the C-C bonds with little change in energy • This allows the out of plane carbons to move into the plane and another to move out. • This conformation is called the envelop conformation. Conformations of Cyclohexane • There is considerable evidence that the most stable conformation of cyclohexane is the Chair conformation. • It is non-planar and all carbon-carbon bond angles are 109.5o, therefore, no angle strain! • There is also no torsional strain since all bonds are staggered. • Plus, the hydrogens atoms at opposite ends are maximally separated Conformations of Cyclohexane • By partial rotation about the C-C bonds, the ring can assume another conformation called the boat conformation • Like the chair, the boat is also free from angle strain • But the boat is not free of torsional strain, some of the C-H bonds are eclipsed. • Additionally, 2 of the H’s are close enough to cause van der Waals repulsion Conformations of Cyclohexane • These repulsions have been called the Flagpole interactions of the boat conformation • Combined, the torsional strain and the flag pole interactions cause the boat to have considerably higher energy than the chair • Although it is more stable, the chair is more rigid than the boat • The boat is quite flexible Conformations of Cyclohexane • This flexibility allows the boat to form a new conformation called the twisted boat. • The twisted boat relieves some torsional strain and reduces the flag pole interactions, but is still 21 kJ/mol more unstable than the chair. • Because of the stability of the chair, more than 99% of the molecules are in the chair conformation at any given moment Positions on a Chair • Six membered rings are the most common in nature so we will pay special attention to them • In the chair, there are two types of H’s: • Equatorial hydrogens- lie around the perimeter of the ring • Axial hydrogens- lie perpendicular to the ring, above and below the ring Positions on a Chair • The cyclohexane rings flips back and forth between two equivalent chair conformations • When the ring flips, all bonds that were axial become equatorial and vice versa • If one of the H’s is replaced with a substituent, the two chair conformations are no longer equal Methyl Cyclohexane • There are two possible chair conformations • The reason for this is due to the 1,3-diaxial interactions • This is created by the steric hindrance when the methyl is in the axial position 1,3-diaxial interactions • The strain caused by the 1,3-diaxial interaction is the same as the strain caused by the close proximity of the methyl group in the gauche form of butane. • Recall that the butane-gauche interaction was equal to 3.8 kJ/mol • When we look at a newman projection of methyl cyclohexane with the methyl in the axial position, we see that there are actually two butane-gauche interactions 1,3-diaxial interactions • There are 2 butane-gauche interactions so that is a total of 7.6 kJ/mol • As the size of the substituent increases, the energy difference increases as well • Example- t-butyl cyclohexane Disubstituted cycloalkane: Cis/Trans Isomerism • The presence of two substituents on the ring of cycloalkane allow for the possibility of cis/trans isomerization • Consider 1,2-dimethylcyclopentane • The cis/trans versions are stereoisomers, that is, they differ only in arrangement of their atoms in space. Disubstituted cycloalkane: Cis/Trans Isomerism • Unlike conformational stereoisomers, they can not be interconverted without breaking bonds • As a result, cis/trans isomers can be separated, placed in separate containers, and stored indefinitely. • 1,3-dimethyl cyclopentane show cis/trans isomerism as well Disubstituted cycloalkane: Cis/Trans Isomerism • Cyclohexane will also show cis/trans with 1,2; 1,3; and 1,4 substitutions patterns. • Problem, these flattened rings do not accurately represent the conformations • Deciding cis/trans in a chair conformation is sometime difficult. Cis/trans in a Chair • Three ways: 1) Flatten the ring 2) Recognize that there is an upper bond and a lower bond on each carbon of the chair 3) Conversion Key Apply what we know! • Knowing what we know now, we can look at a name and determine which is the major conformation • Ex 1- cis-1,4-dimethylcyclohexane • Ex 2- trans-1-t-butyl-3-methylcyclohexane Skipping section 4.14 • Deals with bicyclorings Chemical Reactions of Alkanes • Alkanes as a class are usually characterized by the general inertness to most chemical reagents • Reasons: 1) C-C and C-H bonds are quite strong and require very high temperatures to break 2) C and H have nearly the same electronegativity so the bonds are not polarized 3) No unshared electrons to react Chemical Reactions of Alkanes • There are exceptions: • Alkanes react with Oxygen in combustion reactions • They also react with halogens which we will see in Chapter 10. Synthesis of Alkanes 1) Hydrogenation of Alkenes and Alkynes Double bonds and triple bonds can be reduced to the alkane with H2 gas in the presence of a metal catalyst such as Pt, Pd, or Ni Ex. These reactions are usually done with an alcohol solvent such as ethanol. Synthesis of Alkanes • Not in Book!! 2) Reduction of Alkyl Halides Most alkyl halides react with Zn and aqueous acid to produce an alkane Ex. Synthesis of Alkanes • Not in Book! 3) Alkylation of terminal alkynes We can replace acetylenic H’s with an alkyl group. This is called Alkylation and is very useful in synthesis! We must first remove the H with a strong base. NaNH2 is usually used. Synthesis of Alkanes • The anion can then be combined with a methyl halide or a primary alkyl halide provided that there is no branching at the beta carbon. • Ex. Synthesis of Alkanes • If the alkyl halide used is a secondary, tertiary, or a primary with branching at beta carbon, the reaction gives different products by a mechanism called elimination, which will be discussed in chapter 7 Sample Problem • Synthesize 2-methylpentane from 3-methyl-1butyne and bromomethane. General Principles of Structure and reactivity • The alkylation of the alkynide anion illustrates several essential aspects of structure and reactivity • First, the preparation of the alkynide anion involves simple Bronsted-Lowry acid-base chemistry • Once formed the anion is a lewis base with which the alkyl halide reacts as an electron pair acceptor (lewis acid) General Principles of Structure and reactivity • The anion can be called a nucleophile because of the negative charge concentrated at the terminal carbon • Conversely, the alkyl halide can be called an electrophile because of the partial positive charge at the carbon bearing the halogen • In Summary, many of the reactions in organic chemistry involves acid-base transformations and interactions of reagents with complementary charges. Introduction to Organic Synthesis • Organic Synthesis is the process of building organic molecules from simpler precursors. • Reasons for Organic Synthesis: 1) Create compounds for specific use, ie drugs 2) Create molecules to prove or disprove a hypothesis. Ie usually labeled with D, T or 13C • Very simple organic synthesis may only involve one or two chemical reactions, while others often require many more Introduction to Organic Synthesis • Synthesis of vitamin B12 took 11 years, nearly 100 people and more than 90 step! • An organic synthesis typically involves two types of transformations: 1) Reactions that convert functional groups from one to another 2) Reactions that create new C-C bonds Introduction to Organic Synthesis • We have covered examples of both! • Retrosynthesis- process of starting with the target molecule, identifying precursors, then finding how to make precursors, till you get to readily available starting materials.