Introduction to Hydrocarbons: Alkanes Structure

Introduction to Hydrocarbons

• Science and Fun provides engaging education in science.
• Hydrocarbons are a significant chapter with substantial content.
• This detailed explanation aims to provide a comprehensive understanding.
• While materials exist, reactions are handwritten for better learning.
• This is the first organic chemistry chapter involving reactions.

Chapter Flow in Organic Chemistry

• Basic Study: Covers structure, IUPAC naming (already covered for hydrocarbons).
• Preparation: Describes how to synthesize a particular compound (e.g., alkanes).
• Physical Properties: Includes melting point, boiling point, solubility, smell, color, test.
• Chemical Properties: Reactions the compound undergoes (can vary greatly in number).
• Usags: Applications of the compound (sometimes).
• The chapter will cover alkanes, alkenes, alkynes, and benzene (cyclic compounds).
• Organic chemistry requires memorization, unlike physics or thermodynamics.

Alkanes: Structure

• Carbon atoms in alkanes are sp3 hybridized
• This results in a tetrahedral shape around each Carbon atom.
• Each carbon atom is bonded to four other atoms (either carbon or hydrogen).

Alkanes: Conformations

• Conformations refer to different spatial arrangements of a molecule.
• Ethane (CH3CH3) conformations are discussed, involving rotation around the single bond
• Two main representations: Sawhorse projection and Newman projection.
• Sawhorse Projection: Visualizes molecule from an oblique angle, showing all bonds.
• Newman Projection: Views molecule directly along a carbon-carbon bond axis.
• Eclipsed Conformation: Hydrogens on adjacent carbons are aligned (behind each other).
• Staggered Conformation: Hydrogens on adjacent carbons are as far apart as possible.
• Propane conformations are similar, with one methyl group attached to a carbon.
• Butane conformations are more complex due to the larger size of alkyl group.

Alkanes: Preparation from Unsaturated Hydrocarbons

• Alkenes and alkynes (unsaturated) can be converted to alkanes (saturated) by adding hydrogen.
• This process is called hydrogenation
• Alkenes (double bonds) react with hydrogen (H2) in the presence of a catalyst.
• Common catalysts: Nickel (Ni), palladium (Pd), or platinum (Pt).
• Example: CH2=CH2 + H2 (Ni catalyst) → CH3CH3 (ethane).
• Alkynes (triple bonds) can be hydrogenated in one or two steps to form alkanes.
• One step produces an alkene, the second step saturates the compound to an alkane.

Alkanes: Preparation from Alkyl Halides (Haloalkanes)

• Zinc Metal: Alkyl halides react with zinc metal in the presence of an acid (H+).
• Example: CH3CH2Cl + H2 (Zn, H+) → CH3CH3 + HCl
• Wurtz Reaction: Alkyl halides react with sodium (Na) metal in dry ether solvent.
• Reaction results in the coupling of two alkyl groups.
• General equation: 2 R-X + 2 Na (dry ether) → R-R + 2 NaX.
• Example: 2 CH3Cl + 2 Na (dry ether) → CH3CH3 + 2 NaCl.
• The Wurtz reaction cannot be used to prepare alkanes with an odd number of carbons.

Alkanes: Preparation from Carboxylic Acids

• Decarboxylation: Sodium salts of carboxylic acids are heated with soda lime (NaOH + CaO).
• Process removes a carboxyl group (COOH) as CO3
• Example: CH3COONa + NaOH (CaO, heat) → CH4 + Na2CO3.
• Kolbe's Electrolysis: Electrolysis of aqueous solutions of sodium or potassium salts of carboxylic acids.
• Produces an alkane with twice the number of carbon atoms as the starting acid.
• Example: 2 CH3COONa + 2 H2O (electrolysis) → CH3CH3 + 2 CO2 + H2 + 2 NaOH.
• The reaction involves loss of carbon dioxide (CO2) and formation of alkane by coupling.

Alkanes: Physical Properties

• Approximately Nonpolar: Due to similar electronegativity of carbon and hydrogen.
• Insoluble in Water: Nonpolar nature makes them insoluble in polar solvents like water.
• Soluble in Nonpolar Solvents: Soluble in benzene, aldehydes, ketones, alcohol.
• Weak Intermolecular Forces: Only have weak van der Waals forces of attraction.
• Boiling Point Trend: Increases with increasing size and mass (number of carbons).
• Physical State: First four members (methane to butane) are gases
• Fifth to seventeenth members are liquids
• Higher members are solids.
• Isomers: Branching decreases the melting point and boiling point due to reduced surface area and weaker intermolecular forces.
• Colorless and Odorless: Alkanes are generally colorless and odorless.

Alkanes: Chemical Properties - Free Radical Halogenation

• Reaction involves the substitution of a hydrogen atom by a halogen atom.
• Requires ultraviolet (UV) light to initiate the reaction by forming free radicals.
• Multiple substitutions can occur, leading to a mixture of products.
• Example: CH4 + Cl2 (UV light) → CH3Cl + HCl; CH3Cl + Cl2 (UV light) → CH2Cl2 + HCl; and so on.
• Products include chloromethane, dichloromethane, trichloromethane (chloroform), and tetrachloromethane (carbon tetrachloride).
• Chloroform (CHCl3) was formerly used as an anesthetic, now has other applications.

Alkanes: Chemical Properties - Combustion

• Alkanes react with oxygen (O2) in combustion to produce carbon dioxide (CO2) and water (H2O).
• Large amounts of heat and light are released.
• General equation: CnH2n+2 + O2 → CO2 + H2O.
• Alkanes burn with a blue flame
• Alkenes/alkynes burn with a yellow flame.
• Alkanes are good fuels.

Alkanes: Chemical Properties - Controlled Oxidation

• Oxidation reactions can be controlled using specific catalysts and conditions.
• Methane (CH4) reacts with oxygen in the presence of copper (Cu) catalyst at high temperature and pressure to produce methanol (CH3OH).
• Methane can also react with oxygen in the presence of molybdenum oxide (Mo2O3) to give methanal (HCHO).
• These reactions are highly dependent on the conditions and catalyst used.

Alkanes: Chemical Properties - Isomerization

• Alkanes can be converted to their isomers by heating in the presence of anhydrous aluminum chloride (AlCl3) and hydrogen chloride (HCl).
• Example: Hexane is converted to branched isomers like 2-methylpentane and 3-methylpentane.

Alkanes: Chemical Properties - Aromatization and Reforming

• Alkanes with six or more carbon atoms can be converted to aromatic compounds by heating in the presence of catalysts.
• Catalysts: Oxides of vanadium, chromium, or molybdenum.
• This process is also called dehydrogenation.
• Example: Hexane converts to benzene by removing four molecules of hydrogen (4 H2).

Alkanes: Chemical Properties - Reaction with Steam

• Alkanes react with steam (H2O in gas form) at high temperatures in the presence of a nickel catalyst.
• Produces carbon monoxide (CO) and hydrogen gas (H2).
• Example: CH4 + H2O (Ni, heat) → CO + 3 H2.

Alkanes: Chemical Properties - Pyrolysis (Cracking)

• Pyrolysis involves heating alkanes to high temperatures in the absence of oxygen.
• Leads to the breakdown of larger alkanes into smaller alkanes and alkenes.
• Example: Hexane is heated to produce smaller alkanes and alkenes.
• This is also known as cracking.
• The products vary depending on the conditions.

Alkenes: Structure

• Carbon bonded with a double bond is sp2 hybridized
• Trigonal planar shape
• Ethyne CH2=CH2 can be represented with two hydrogen atoms each
• The carbon is sp3 hybridized

Alkenes: Geometric Isomers (Cis/Trans)

• Geometric isomers is based on arrangement around the double bond which have restricted rotation.
• Cis Isomers: Similar groups are on the same side of the double bond.
• Trans Isomers: Similar groups are on opposite sides of the double bond.
• Example: But-2-ene (CH3CH=CHCH3) exists in cis and trans forms.
• Nonpolar trans are dipole moment = 0
• Polar trans are dipole moment is non-zero = about .33
• A Trans molecules has a higher melting point and higher boiling point due to attraction with a higher dipole.
• Geometric isomers don't work with ethyne due to the identical structure

Alkenes: Preparation from Alkynes

• Alkynes can be reduced to alkenes by controlled hydrogenation.
• Reaction requires hydrogen (H2) and a metal catalyst (Ni, Pd, Pt).
• Example: Ethyne + Hydrogen (H2 in Ni form) => Ethene

Alkenes: Preparation From Alkyl Halides (Dehydrohalogenation)

• Alkyl halides can be converted to alkenes by dehydrohalogenation.
• Involves removal of a hydrogen atom and a halogen atom from adjacent carbon atoms. - Requires alcoholic potassium hydroxide (alcoholic KOH) as the reagent.
• It's a Beta elimination in order to eliminate both hydrogen via KOH for the alcohol.
• Alpha Carbons attach to a functional group dirrectly
• Beta Caribon's attach to alpha carbons.

Alkenes: Preparation from Alkyl Halides (Saytzeff's Rule)

• In dehydrohalogenation, if there are multiple beta-hydrogens available, Saytzeff's (Zaitsev’s) rule is applied.
• Saytzeff's Rule: The preferred product is the more substituted alkene (more alkyl groups attached to the double-bonded carbons).
• With Beta elimination any products with major alkyl group produce higher level of product

Alkenes: Preparation from Vicinal Dihalides

• Vicinal dihalides can be converted to alkenes by treatment with zinc metal (Zn).
• Reaction involves removal of halogen atoms from adjacent carbon atoms. Example: dihalide + Zinc in a Br solution will yield Zinc, Br and ethyne

Alkenes: Preparation from Alcohols (Dehydration)

• Alcohols can be converted to alkenes by dehydration.
• Involves removal of a water molecule (H2O) from the alcohol molecule.
• Requires concentrated sulfuric H2SO4
• Reaction is typically carried out at high temperatures (443 K).

Alkenes: Physical Properties

• Polarity: Again, the polar trend follows: gases, liquids, solids for all states
• Most members are in gas though
• Also color-less and the boiling-point are high
• Also are soluable in non-polar
• Otherwise, not much unique

Alkenes: Chemical Properties - Addition of Hydrogen (as described earlier in Alkanes)

• Hydrogen + alkene = Alkane

Alkenes: Chemical Properties - Addition of Water

• h2o -> OH
• Requires H+
• It is a Beta-Elmination

Alkenes: Chemical Properties - Vinyl Alcohol

• Vinyl is the alcohol that is bonded to the double bond

Alkenes: Chemical Properties - Markonikov and Anti-Markonikov Rule

• Symmetrical Alkenes are symmetrical no matter how you alter the bond
• Anti-symmetrical alkenes are not , which means some products are different if you alter bonds

Alkenes: Chemical Properties - Markonikov Rule

• Hydrogen, H, goes where there is more hydrogen
• And then x and Cl goes

Alkenes: Chemical Properties - Anti - Markonikov Rule

• H goes where are less hydrogen atoms and then will have that bond more

Alkenes: Chemical Propertities - Ozonolysis Reaction

• Sigma and Pi bond both will break when you add ozone
• The bond usually makes an oxide that is very stable
• Zinc will break two bonds and produce Zn oxide + other stuff

Alkenes: Chemical Propertities - Types of o-lists

Alkenes: Chemical Properities - Polymerization

Alkenes: Chemical Propertities - Alkenes - Polymizeration Reaction

• Requires a catalyst
• Pi bond will break and attach to the surrounding one to bond
• Just keeps going in the same chain

Alkynes: Introduction

• At least one carbon-carbon triple bond
• Carbon is Highbridiszed by EP for this molecule.

Alkynes: Linear/Structure

• Carbon bonded to a double bond is sp2 hybridized
• Linear shape, with one triple bond

Alkynes: Linear/Structure - Preparation From Dihalides

• The halogen is removed by ethonal KOH
• Beta elimination occurs
• You do it twice, to finally yeild the ethyne

Alkynes: Linear/Structure - Preparation From Calcium Carbede

Alkynes: Physical Properties

• Gas liquid solid polar
• Just depends on molecular weight.
• Otherwise normal properties
• Like soluble normal

Alkynes: Preparation Chemical

• If u put water and acid it happens very differently
• Pi shift which causes alcohol.

Alkynes: Chemical Properties - Tautomerism

• This is the process of having the H change its location from one side of the other

Alkynes: Chemical Properties - Acidic Properties

• Acidic property is the percentage of high character. The higest character is the lowest one.

Alkynes: Chemical Properties - Acidic Properties

• H bonds with other Metals

Alkynes: Chemical Properties - Polymers

Alkynes: Chemical Properties - Linear

• Again, double bond between H

Alkynes: Chemical Properties -Cyclic

• Ethyne bonds with the carbon atoms to make benzene and other aromatic compunds

More to come

Aromatic Compounds: Properties and Rules

• Aromatic and fragrant aromatic does things.
• Now any compound which are all full and meet all requirements

Aromatic Compounds: Hackle Rule

• Carbon as double bond for everything in the molecule.
• Num pi E equals 4 N+2 Where n=Num rings.

Review: Can It Be Aromatic?

• Examples are listed.

Preparation of Benzene:

By cyclic polymerization

• Create benxene by linking three chains

Preparation of Benzene: - Decarboxilation With Benene

• Remove one cooh, one OH

Preparation of Benzene: -Using Phenol

• Zinc will remove oxegon and add an H to

Benzene: Physical Properties

• Benzine in non-polar
• Doesn't like the water water
• But likes the other chemicals
• As we know benzine smells good so it can repel bugs and get rid of other things

Benzene: Chemical Properties

• Benzene is bad
• Bends in the world

- Addtion with x2

• Now it attaches the halogen all the time and then it can now be

Addition REACTION

• And now u can add halogen with UV

Summary

• So toxic the molecules like to use the oxegen and will then be the end
• Hope u now understand thanks

Explore the structure and properties with handwritten reactions for better learning. The chapter covers alkanes, alkenes, alkynes and benzene. Organic chemistry requires memorization.