3. Organic Compounds: Alkanes and Cycloalkanes

2 Classification of Organic Compounds Organic compounds are classified into different types based on the kinds of functional groups present in its structure A functional group is an atom or groups of atoms that determine the types of chemical reactions that the compound participates in. For example, the double bonds in simple and complex alkenes react with bromine in the same way

McMurry Organic Chemistry 6th edition Chapter 3 (c) Types of Functional Groups: Multiple Carbon–Carbon Bonds Alkenes have a C=C double bond Alkynes have a C  C triple bond In Arenes or aromatic hydrocarbons, the functional group is the benzene ring

McMurry Organic Chemistry 6th edition Chapter 3 (c) Functional Groups with Carbon Singly Bonded to an Electronegative Atom Alkyl halide: C bonded to halogen (C-X) Alcohol: C (part of aliphatic group) bonded O of a hydroxyl group (C-OH) Phenol: C (part of aromatic group) bonded O of a hydroxyl group (C-OH) Ether: Two C’s bonded to the same O (C-O-C) Amine: C bonded to N (C-N) Thiol: C bonded to SH group (C-SH) Thioether: Two C’s bonded to same S (C-S-C) Bonds are polar, with partial positive charge on C (  +) and partial negative charge (  ) on electronegative atom

McMurry Organic Chemistry 6th edition Chapter 3 (c) Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups) Aldehyde: one hydrogen bonded to C=O Ketone: two C’s bonded to the C=O Carboxylic acid:  OH bonded to the C=O Ester: C-O bonded to the C=O Amide: C-N bonded to the C=O Acid chloride: Cl bonded to the C=O Carbonyl C has partial positive charge (  +) Carbonyl O has partial negative charge (  -).

6 3.2 Alkanes and Alkane Isomers Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups) The general molecular formula of an alkane with n C atoms is C n H 2n+2 When n  4, more than one structural formula can be drawn for the same molecular formula, C n H 2n+2 Compounds with the same molecular formula but with different structural formulas are called structural isomers or constitutional isomers. Structural isomers have different physical properties (Boiling points, melting points, densities etc.) Alkanes are called saturated hydrocarbons because they are saturated with hydrogen (no more can be added Alkanes, Alkenes, Alkynes, Cycloalkanes, Cycloalkenes and Cycloalkynes are also classified as aliphatic hydrocarbons

7 Alkane Isomers CH 4 = methane, C 2 H 6 = ethane, C 3 H 8 = propane Isomers of alkanes are of 2 types: straight chain or normal alkanes and branched chain alkanes. Straight-chain or normal alkanes have all n C atoms attached in a single connected chain. That is, no C atom is attached to more than 2 other C atoms. For any alkane with n  4, there is only one straight chain isomer. Branched-chain alkanes have one or more C atoms connected to 3 or 4 other C atoms. C 4 H 10 has one branched isomer. For all alkanes with n > 4, there are more than one branched isomers.

McMurry Organic Chemistry 6th edition Chapter 3 (c) Examples of Constitutional Isomers Compounds other than alkanes can be constitutional isomers of one another They must have the same molecular formula to be isomers

McMurry Organic Chemistry 6th edition Chapter 3 (c) Condensed Structures of Alkanes We can represent an alkane in a brief form or in many types of extended form A condensed structure does not show bonds but lists atoms, such as CH 3 CH 2 CH 3 (propane) CH 3 C(CH 3 ) 2 CH 3 (IUPAC: 2,2-dimethylpropane; common: neopentane)

McMurry Organic Chemistry 6th edition Chapter 3 (c) Names of Small Hydrocarbons No. of CarbonsFormula Name(C n H 2n+2 ) 1MethaneCH 4 2EthaneC2H6C2H6 3PropaneC3H8C3H8 4ButaneC 4 H 10 5PentaneC 5 H 12 6HexaneC 6 H 14 7HeptaneC 7 H 16 8OctaneC 8 H 18 9NonaneC 9 H 20 10DecaneC 10 H 22

McMurry Organic Chemistry 6th edition Chapter 3 (c) Alkyl Groups Alkyl group – remove one H from an alkane (a part of a structure) General abbreviation “R” (for Radical, an incomplete species or the “rest” of the molecule) Name: replace -ane ending of alkane with -yl ending CH 3 is “methyl” (from methane) CH 2 CH 3 is “ethyl” from ethane See Table 3.4 for a list

McMurry Organic Chemistry 6th edition Chapter 3 (c) Types of Alkyl groups Classified by the connection site (See Figure 3.3) a carbon at the end of a chain (primary alkyl group) a carbon in the middle of a chain (secondary alkyl group) a carbon with three carbons attached to it (tertiary alkyl group)

Naming Branched Alkanes Find the longest continuous chain of C atoms. This is the parent chain. Name the parent chain Number the C atoms in the parent chain starting from the end closest to a substituent Substituents are atoms (Cl, Br, I) or groups of atoms (examples: alkyl groups, OH, carbonyl group, amino group) that have replaced H’s Specify the number(s) of the C atom(s) to which the substituent(s) is(are) attached to and the name(s). Letters and numbers are separated by hyphens List the substituents in alphabetical order before the parent name. When there are more than 1 substituent of the same type, use prefixes di, tri, tetra etc. to specify the number of each type. The location of each substituent (even when they are identical and attached to the same C) should be specified.

McMurry Organic Chemistry 6th edition Chapter 3 (c) If two different longest chains are possible, choose the one with the more substituents as the parent chain If two different substituents are equidistant from either end, chose to number from that end that is closer to the substituent that is alphabetically first. The prefixes di-, tri-, tetra- are ignored when determining alphabetical order. The prefixes sec- and t- or tert- are also ignored. But cyclo, iso are considered. In naming complex substituents, any prefixes di-, tri- ARE NOT IGNORED IN DETERMINING ORDER. For example if we call t- butyl 1,1-dimethylethyl instead, the di is considered in determining the order.

McMurry Organic Chemistry 6th edition Chapter 3 (c) Drawing structural isomers for alkanes with n≥4 C atoms: Start with parent straight chain with n≥3. Replacing any H attached to a secondary C with a methyl group will result in a branched isomer with n+1 C atoms. Replacing any H attached to either of the two primary C atoms of a straight chain alkane will only yield the corresponding straight chain alkane with n+1 C atoms. When drawing isomers, replacing any of the H on symmetrically positioned C atoms with the same alkyl group will yield the same isomer. Verify by naming your isomers. If the names of two isomers are the same, they are identical. Drawing Branched Isomers

Properties of Alkanes Alkanes undergo substitution reactions and combustion reactions (alkane + O 2 → CO 2 (g) + H 2 O) In substitution reactions, H’s are replaced by other atoms. For example,alkanes react with Cl 2 in the presence of light to replace H’s with Cl’s. The boiling points & melting points of alkanes increase with increasing chain length. Among the structural isomers of an alkane, the less branched the alkane, higher its boiling point, melting point.

Cycloalkanes Cycloalkanes are alkanes that have carbon atoms attached to form a ring In simple cycloalkanes, all n C atoms are attached to form a ring. The general molecular formula of a cycloalkane with n C atoms is C n H 2n The condensed structural formula of a simple cycloalkane with n C atoms is a regular polygon with n vertices The boiling points of cycloalkanes also increases with increase in ring size. Cycloalkanes exhibit structural isomerism. A cycloalkane has the same general formula as an alkene with the same number of C atoms and 1 double bond. In addition, any cycloalkane with more than 3 C atoms will also have cyclic structural isomers cyclopropane cyclohexane cyclopentane cyclobutane

McMurry Organic Chemistry 6th edition Chapter 3 (c) Complex Cycloalkanes Naturally occurring materials contain cycloalkane structures Examples: chrysanthemic acid (cyclopropane), prostaglandins (cyclopentane), steroids (cyclohexanes and cyclopentane)

McMurry Organic Chemistry 6th edition Chapter 3 (c) Naming SubstitutedCycloalkanes Count the number of carbon atoms in the ring and the number of C atoms in the largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent chain, the compound is named as an alkyl-substituted cycloalkane. Otherwise, if the number of C atoms is greater in the alkyl chain, the compound is named as a cycloalkyl-substituted alkane There is no need to number the ring if there is only 1 substituent. When there are more than 1 substituent, the ring has to be numbered. For an alkyl- or halo-substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring so that the second substituent has as low a number as possible. The alphabetically first substituent is given a lower number Number the substituents and write the name

Cis-Trans Isomerism in Cycloalkanes Stereoisomers have the same molecular formula, the same connectivity but different 3-D orientations of substituents. The rotation about C-C bonds in cycloalkanes is limited which results in stereo isomerism when there are 2 or more substituents. Rings have two “faces” and the relative positions of two different substituents will result in different stereoisomers. In disubstituted cycloalkanes, the isomer with both substituents on the same side is called the cis isomer and the isomer with substituents on opposite sides is called the trans isomer. For example, consider the 2 stereoisomers of 1,2-dimethyl- cyclopropane. Cis-1,2-dimethylcyclopropane has both methyl substituents on the same side. Trans-1,2-dimethycyclopropane has the methyls on opposite sides of the ring

McMurry Organic Chemistry 6th edition Chapter 3 (c) Structural Isomers & Stereoisomers

McMurry Organic Chemistry 6th edition Chapter 3 (c) Naming Bicycloalkanes & Spiroalkanes 1.A bicycloalkane consists of 2 rings that share 2 C atoms. The two shared C atoms are called ‘bridgehead C’s’. There are 3 paths or ‘bridges’ that connect these two bridgehead C’s. 2.The parent name of a bicycloalkane is that of an unbranched alkane having the same number of carbons with a prefix bicyclo. 3.Begin numbering at one of the two "bridgehead" carbons. Proceed around the longest bridge to the 2nd bridgehead, then around the next longest bridge and then finally around the shortest bridge. 4.If the one or more substituents are present, start numbering from the bridgehead which results in the first substituent on the larger bridge having a smaller number. 5.The length of a ‘bridge’ = number of C in that bridge between the bridgehead C’s and not including the bridgehead C’s. 6.The lengths of the bridges are specified within square brackets between the word bicyclo and the parent name. The lengths are written in descending order and separated by periods. For example, bicyclo[4.2.1]nonane has 9 C atoms two of which are bridgehead C’s and therefore common to all 3 bridges. The 3 bridges have 4 C, 2C and 1 C respectively.

McMurry Organic Chemistry 6th edition Chapter 3 (c) Naming Spiroalkanes 1.A spiroalkane consists of 2 rings that share 1 C atom. 2.The parent name of a spiroalkane is that of an unbranched alkane having the same number of carbons with a prefix spiro. 3.Count the C’s in the smaller ring and the C’s in the larger ring to determine the bridge lengths. Do not include the shared C in determining the bridge lengths. 4.If substituents are present on the ring, numbering of the rings starts in the small ring on the C next to the shared C, passes round this ring and through the shared C into the larger ring. The numbering goes round the two rings in the direction which produces the lowest numbers in each ring.. 5.The lengths of the bridges are specified within square brackets between the word spiro and the parent name. 6.The lengths are written in ascending order and separated by periods. For example, spiro[3.4]octane has 8 C atoms one of which is a bridgehead C and therefore common to both rings. The 2 bridges have 3 Cand 4 C respectively. Check the following site for problems: