Cycloalkanes Other Than Cyclohexane Have High Heats of Combustion per CH2 Group because of

Organic Chemistry Alkanes & Cycloalkanes Notes Organic Chemistry – Alkanes & Cycloalkanes NOTES - Alkanes (Aliphatic Hydrocarbons): - alkane—hydrocarbon compound (contains only C and H) that also contain ONLY SINGLE bonds - also called saturated hydrocarbons (because they contain the maximum amount of H that will fit on the carbon chain - also known as aliphatic hydrocarbons - General Formula: CnH2n+2 - alkanes generally come from fossil fuels - Naming Alkanes: - names of alkanes serve as the foundation for naming everything else in organic chemistry - named for the number of carbons in the chain with the ending “-ANE” - notice that the chains get longer by adding “-CH2-“ to the middle and there are always “CH3”s on the ends Image courtesy Organic Chemistry Alkanes & Cycloalkanes Notes - Alkyl Groups (Side Chains): - alkyl groups—small alkane chains that “branch off” from the main chain - these side groups are typically named after the root alkane but changing the ending to “-YL” Image courtesy - the prefix “ISO-“ means the same or equal (branch attachment at the MIDDLE of the chain) - the prefix “N-“ (normal) refers to a STRAIGHT CHAIN - there are also varying degrees of substitution: 1) PRIMARY (1o) carbon—bonded to only ONE other carbon atom (this carbon is always at the END of the chain!) 2) SECONDARY (2o) carbon—bonded to 2 other carbon atoms (in the middle of the chain) {PREFIX → “SEC-“} 3) TERTIARY (3o) carbon—bonded to 3 other carbon atoms (in the middle of the chain with a branch attached to it) {PREFIX → “TERT-“ or “t-“} 4) QUARTERNARY (4o) carbon—bonded to 4 other carbon atoms Organic Chemistry Alkanes & Cycloalkanes Notes - IUPAC Names for Alkanes: - PARENT CHAIN—the longest unbroken chain of carbon atoms in the molecule - SUBSTITUENT—generic term for any group that does NOT belong to the main chain **NOTE: Right now, substituent will only refer to alkyl groups but in later chapters this could be a lot of other groups as well!!** 1) Root name for the alkane is the name of the PARENT CHAIN 2) Assign NUMBERS to each carbon in the PARENT CHAIN starting at the side that is CLOSEST to the first point of branching - this may NOT always be left → right!! 3) Identify the names of the SUBSTITUENT groups and NUMBER them according to their position on the parent chain 4) Write the names of the substituents ALPHABETICALLY followed by the name of the parent alkane Ex. Name this molecule according to IUPAC rules: Ex. Draw the line drawing for 2,3,4-trimethylheptane - Constitutional Isomerism: Organic Chemistry Alkanes & Cycloalkanes Notes - constitutional isomers—compounds which have the SAME molecular formula but have different arrangements of atoms - structure and properties of each compound are very different - Conformations of Alkanes: - alkanes contain only σ bonds (SINGLE!) - a C—C single bond is FREELY ROTATING - if we consider the structure of ETHANE (C2H6) we can see that the C—C bond freely rotates to give 2 possible conformations - staggered conformation—H atoms are 60o rotated away from the plane of the H atoms on the other C atom - eclipsed—H atoms line up in the same plane as the H atoms on the other C atom - Newman Projections—give the point of view of looking down the C—C so you may see how the H line up against each other - molecules such as ethane prefer the STAGGERED conformation because there is less repulsion between electron clouds in this configuration when compared to the higher TORSIONAL STRAIN of the eclipsed conformation - this becomes even more true when there are larger substituents than just H atoms on the carbons!! - Physical Properties of Alkanes: - alkanes are NONPOLAR (hydrophobic!) - have relatively LOW melting and boiling points in comparison with other Organic Chemistry Alkanes & Cycloalkanes Notes molecules - this is because there are only London Dispersion forces holding them together - the bigger the alkane molecule, the HIGHER the melting and boiling points - bigger molecules will have stronger attractions to each other which will make it harder to pull them apart in order to melt or boil - Reactions of Alkanes: - alkanes are not tremendously reactive - major reactions that alkanes undergo are combustion reactions (burning with O2 to release heat energy) - the products from this reaction are CO2 and H2O vapor CnH2n+2 + O2 → CO2 + H2O + energy - Cycloalkanes: - cycloalkanes—alkanes in which the carbons are connected to form a ring structure (still all SINGLE bonds!) - General Formula: CnH2n - Naming Cycloalkanes: - named just like alkanes but the prefix “CYCLO-“ is added to the parent ring chain - substituent groups are named simply by putting the name of the side Organic Chemistry Alkanes & Cycloalkanes Notes group in front of the parent alkane - Ex. propylcyclohexane - if there are 2 groups substituted on the ring, then rank them in alphabetical order - the C attached to the first group (alphabetically) is given the number “1” and the rest of the carbons in the ring are numbered in the direction that gives the LOWEST number to the C attached to the second group (alphabetically) - Ex. 1-ethyl-3-methylcyclohexane - for 3 or more groups, follow the same rules as for naming alkanes - Name the following molecule: - Draw the structure (line drawing) for 1,1,2-trimethylcyclobutane - Cis-Trans Isomerism in Cycloalkanes: - cis-trans isomerism—all atoms in the molecule are bonded to each other Organic Chemistry - Alkanes & Cycloalkanes Notes in the SAME pattern but point in DIFFERENT directions these are NOT resonance structures because they differ in more than just single/double bonds the prefix “CIS-“ means on the SAME side the prefix “TRANS-“ means on OPPOSITE sides (ACROSS) Ex. 1,2 dimethylcyclopentane: - Ring Strain: - ring strain—a measure of instability in a cyclic ring structure due to angle strain caused by unfavorable C—C bond angles and torsional strain (eclipsing substituents on adjacent C atoms) - Ex. Cyclopropane & Cyclobutane - as the ring gets BIGGER (more Cs) the bond angles get BIGGER (so less angle strain!!) - the rings may BEND a bit to relieve torsional strain - Structure of Cyclohexane: Organic Chemistry Alkanes & Cycloalkanes Notes - while cyclopropane, cyclobutane and cyclopentane all have some degree of bond angle strain, cyclopentane is almost the exact desired 109.5o!! - cyclohexane is not flat (or the bond angles would make it very unstable!) - cyclohexane forms either the “CHAIR” or “BOAT” conformation!! - these 2 conformations may be converted into each other but the CHAIR conformation is strongly favored - in the CHAIR conformation there is NO torsional strain because all of the hydrogens are STAGGERED and the hydrogens are either in the AXIAL position (pointing straight UP and DOWN) or the EQUITORIAL position (in the plane of the ring) and these 2 positions ALTERNATE!! - in the BOAT conformation, there IS torsional strain because the hydrogens are ECLIPSED and the HYDROGENS on carbons 1 and 4 are pointed TOWARD each other which is an UNSTABLE conformation - Monosubstituted Cyclohexanes - if a CH3 or larger group is substituted for one of the H in cyclohexane it is MORE STABLE in the EQUITORIAL position of the CHAIR conformation - there is a tendency for a 1,3-DIAXIAL INTERACTION of the CH3 with the H when it is in the AXIAL position that is LESS STABLE which is known as STERIC HINDRANCE Organic Chemistry Alkanes & Cycloalkanes Notes - steric hindrance (steric strain)—instability caused by two substituent groups bumping into each other (becomes LARGER as the groups get LARGER) - Disubstituted Cyclohexanes: - Ex. cis-1,2-dimethylcyclohexane vs. trans-1,2-dimethylcyclohexane - in the CIS formation ONE CH3 MUST be in the AXIAL position so some STERIC HINDRANCE will always occur and no conformation is favorable over the other - in the TRANS formation, one conformation has BOTH CH3s in the AXIAL and the other conformation has BOTH in EQUITORIAL and the EQUITORIAL conformations are FAVORED!! - Ex. cis-1,3-dimethylcycloalkane - in this molecule the diequitorial conformation will be much more stable because there is a 1,3-diaxial interaction in the diaxial conformation Organic Chemistry Alkanes & Cycloalkanes Notes - LARGE substituents definitely prefer to be in EQUITORIAL positions and NOT AXIAL!!