Alkanes Flashcards

Alkanes

• Alkanes are compounds containing C–C single bonds and C–H bonds.
• Connecting carbons can result in molecules of varying sizes, from small to large.
• They are saturated hydrocarbons.
• Alkanes are also known as aliphatic compounds.
• The general formula for an alkane with no rings is CnH{2n+2}, where 'n' is the number of carbon atoms.

Homologous Series

• A homologous series is one that can be represented by a general molecular formula.
• For aliphatic alkanes, the general formula is CnH{2n+2}.
• Members of a homologous series share the same general formula and exhibit similar chemical properties.
• They demonstrate a gradation of physical properties as their size increases.
• For instance, boiling points and melting points rise with increasing alkane size.
• Straight-chain alkanes are alkanes where each carbon is connected to no more than two other carbons.
• Branched-chain alkanes are alkanes that contain one or more carbon atoms connected to three or four other carbon atoms.

Fractional Distillation of Crude Oil

• Fractional distillation of crude oil yields various products:
  • Natural Gas: C1-C4
  • Gasoline: C4-C{12}, Boiling point (Bpt) 40-200 ºC
  • Kerosene: C{12}-C{16}, Bpt 200-250 ºC
  • Heating Oil: C{15}-C{18}, Bpt 250-300 ºC
• Straight-chain alkanes are desirable as pure fuels, but can cause engine knock.
• n-Heptane has an octane rating of 0.
• 2,2,4-trimethylpentane has an octane rating of 100.
• Catalytic cracking is a process used in refining.

Straight-Chain Alkanes

• The boiling and melting points of straight-chain alkanes generally increase with the number of carbon atoms.

Straight Chain Alkanes

Alkyl Groups

• Alkyl groups are partial structures that remain after the removal of one hydrogen atom from an alkane.
• They are not stable compounds but parts of larger compounds.
• To name them, replace the –ane ending of the alkane with –yl ending.
  • –CH3 is methyl, derived from methane (CH4).
  • –CH2CH3 is ethyl, derived from ethane (CH3CH3).

Straight-Chain Alkyl Groups

Nomenclature

• The International Union of Pure and Applied Chemistry (IUPAC) provides a system for nomenclature.
  • Steps in naming complex branched-chain alkanes:
    1. Identify the longest continuous chain of carbon atoms and use the name of that chain as the parent name.

Naming Alkanes

1. Identify the longest continuous chain of carbon atoms; the name of this chain becomes the parent name.
   • If two different chains of equal length exist, select the one with the greater number of branch points to be the parent chain.
2. Number the atoms in the longest chain.
   • Start numbering at the end nearest the first branch point, numbering each carbon atom in the parent chain.
   • If branching occurs at an equal distance from both ends of the parent chain, begin numbering at the end nearer the second branch point.
3. Substituents
   • Identify and number substituents.
   • Assign a number (locant) to each substituent to specify its attachment point on the parent chain.
   • If two substituents are on the same carbon, give both the same number.
4. Write the name as a single word
   • Examples:
     • 3-Methylhexane
     • 3-Ethyl-4,7-dimethylnonane
     • 4-Ethyl-3-methylheptane
     • 4-Ethyl-2,4-dimethylhexane
     • 3-Ethyl-2-methylhexane
5. Naming branched substituents
   • Name a branched substituent as if it were a compound itself.
     • Example: A 2-methylpropyl substituent
     • Named as a 2,3,6-trisubstituted decane

Worked Example

• Give IUPAC names for the following compounds:
  • a) 2,4-Dimethylpentane
  • b) 2,2,5-Trimethylhexane

Isomers

• Official IUPAC definition of isomers:
  1. They must have the same molecular formula
  2. They differ in some way in the arrangement of their atoms
• Isomers are species (or molecular entities) that share the same atomic composition (molecular formula) but have different line formulas or stereochemical formulas, resulting in different physical and/or chemical properties.

Isomers – the have the same molecular formula, but different structures

• Structural Isomers – same molecular formula, but atoms are bonded in different orders.
  • Isobutane, Isopentane and Neopentane all have the same molecular formula as n-pentane, C5H{12}
  • They have different Physical Properties, Mpt, Bpt, densities
  • C4H{10} – has two isomers, n-butane and isobutane (2-methylpropane) (2,2-dimethylpropane) (2-methylbutane)

Structural Isomers

• Isomerism between structures differing in constitution and described by different line formulae
• Structural (or constitutional) isomers: have the same chemical formula but the atoms are bonded together in a different pattern
  • Examples
    • ethanol (an alcohol)
    • dimethyl ether (an ether)
    • butane
    • 2-methylpropane

Structural isomers – two examples

• Example 1 and 2
  • Same molecular formula - C5H12 Molecular Weight: 72.15
  • Same atom connections  → Structural isomers

Alkane Isomers

• One isomer: CH4 = methane, C2H6 = ethane, C3H_8 = propane
• Molecular formula of an alkane with more than three carbons can give more than one structure
• C_4 (butane) = butane and isobutane
• C_5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane

Worked Example

• Draw structures of the five isomers of C6H{14}
• Solution:
  • Drawing all possible six-carbon skeletons and adding hydrogens to ensure that all carbons have four bonds
  • The six-carbon straight-chain skeleton is drawn
  • Drawing a five-carbon chain, identifying different types of carbon atoms on the chain, and adding a –CH_3 group to each of the different types of carbons, generating two skeletons
    Repeating the process with the four-carbon chain to gives rise to the last two skeletons

Examples

• Structural Formula C5H{10}O_2
  • propyl acetate Chemical Formula: C5H{10}O_2 Molecular Weight: 102,1
  • methyl butyrate Chemical Formula: C5H{10}O_2 Molecular Weight: 102,1

Example: Monosaccharides (Sugars)

• Glucose and fructose are structural isomers
  • GLUCOSE Chemical Formula: C6H{12}O_6
  • FRUCTOSE Chemical Formula: C6H{12}O_6

Isomers

• Be careful not to confuse the same molecule drawn in different orientations
  • 1,1-dichloroethane CHCl2CH3 (bp 57.3 C)
  • 1,2-dichloroethane CH2ClCH2Cl (bp 83.5 C)

Reactions of Alkanes

1. Halogenation: Forms haloalkanes.
2. Oxidation/Combustion: Decomposes into carbon dioxide and water.
3. Dehydrogenation: Forms alkenes.

Oxidation/Combustion

• Hydrocarbons (e.g., alkanes) burn in excess oxygen to produce carbon dioxide and water: alkane + O2 ----> CO2 + H_2O
• For example:
  • CH4 + 2O2 ---> CO2 + 2H2O
  • 2C2H6 + 7O2 ---> 4CO2 + 6H_2O
• This reaction is highly exothermic and spontaneous.
• It is typically ignited by a spark or flame.
• These reactions serve as sources of heat and power.
• Insufficient O_2 can lead to the formation of CO (carbon monoxide) or C (carbon):
  • 2CH4 + 3O2 ---> 2CO + 4H_2O

Halogenation

• CnH{2n+2} + X2 ------------> CnH_{2n+1}X + HX
  • (alkane) (halogen) (haloalkane or alkyl halide)
• For example,
  • CH4 + Cl2 --------> CH_3Cl + HCl
• In the presence of heat, sunlight, or radiation.
• This is a substitution reaction where one H in CH4 is replaced by a Cl from Cl2.
• It is a stepwise process involving initiation, propagation, and termination.
• This three-step process is known as a free radical process.

Reactions of Alkanes

• Combustion: CH4 + O2 -> CO2 + 2H2O + energy
• Dehydrogenation: RCH2CH2R -> RHC=CHR + H_2 (High Temp. catalyst)
• Halogenation – radical substitution reactions: Br_2 + Br -> HBr (light or heat)

Halogenation Substitution Reaction

• a reaction in which part of a small reacting molecule replaces an atom or a group of atoms on the organic molecule
  • Ethane to Bromoethane + HBr (Heat or Light ∆ or hv)
  • Methane to chloromethane + HCl (Heat or Light ∆ or hv), CH2Cl2 and CHCl_3 may be observed

Haloalkanes

• (Alkyl halides)

Organohalides/ Alkyl Halides/ Haloalkanes

• Alkyl halides are organic compounds containing at least one or more halogen atoms.
• Found in abundance in nature
• Alkyl halides comprise a halogen atom bonded to a saturated, sp3- hybridized carbon atom.
• Think of the valency of the halogens!

Halogen Substituents

• Halogen substituents are common on drug molecules.
• Naming:
  • F = fluoro
  • Br = bromo
  • Cl = chloro
  • I = iodo
• Examples
  • fluoxetine
  • chlorpheniramine

Names and structures of alkyl halides

1. Longest chain is named the parent chain
   • Halogen is considered a substituent on the parent alkane chain
2. Carbons of the parent chain are numbered beginning at the end closer to the first substituent
3. When more than one halogen is present, each are numbered and listed in alphabetical order when writing the name
   4. If the parent chain cannot be properly numbered from either end by step 2, numbering begins at the end closer to the substituent that has alphabetical precedence.

Worked Example

• Provide the IUPAC name of the following alkyl halide: CH3CH2CH2CH2I
• Solution:
  • 1-Iodobutane
  • Halogens are treated the same as alkyl substituents but are named as a halo group

Comparison of the Halomethanes

• With progression down the periodic table:
  • Size of halogens increase
  • C–X bond lengths increase
  • C–X bond strengths decrease (X = halogen)

Polarity of C─X bonds

• Halomethanes possess a substantial dipole moment
• Slight positive charge (δ+) on carbon atoms
• Slight negative charge (δ-) on halogens
• C─X carbon atom is considered to behave as an electrophile in reactions (X = halogen substituent)

Cycloalkanes

• Cycloalkanes or alicyclic compounds: Saturated cyclic hydrocarbons
• General formula (CnH{2n})
• Can be represented using skeletal drawings

Chair and boat conformers: Cyclohexane

• Cyclohexane can exist as chair conformers or boat conformers.

Cycloalkanes

• Cyclopropane - CnH{2n}
• Cyclobutane
• Cyclopentane

Organic Compounds: Cyclic

• Large number of organic compounds contain rings of carbon atoms
• Example
  • Prostaglandins
  • Steroids

Cholesterol

• Steroid
• Three cyclohexane rings linked to a cyclopentane ring
• A and D rings adopt chair conformations
• Major constituent of cell membranes

Naming Cycloalkanes

• Examples of naming conventions for cycloalkanes with various substituents.

Worked example

• Draw structural formulae for the following:
  • 2,3-dimethylcyclohexane
  • 2,4,5-trichlorocyclopentane
  • Isopropylcyclohaxane
  • Bromocyclopropane