Addition of Halogens to Alkenes

Addition of Halogens to Alkenes

Alkenes readily react with bromine and chlorine to form 1,2-dihaloalkanes (vicinal dihalides). In this reaction the π bond of the alkene reacts with the halogen to produce two new carbon-halogen σ bonds. The reaction is commonly carried out in an inert solvent such as CH₂Cl₂, CHCl₃, or CCl₄.


The mechanism for the addition of both bromine and chlorine is an electrophilic addition pathway involving an ionic intermediate. For both acyclic and cyclic non-conjugated alkenes this intermediate is a bromonium or chloronium ion. Nucleophilic attack of halide ion on one of the two electrophilic carbons of the bromonium (chloronium) ion gives the product.
The mechanism of the reaction of 2-methylpropene with bromine is shown below.
In the second step of the reaction the incoming bromide ion reacts with carbon on the side opposite the carbon-bromine bond that is being broken. The consequence of this reaction stereochemistry is seen most easily in cycloalkenes where only the trans dihalocycloalkane product is found. The addition of bromine to cyclopentene is a case in point:
In fact, it was the observation that the halogenation of alkenes involves this anti addition (the two halogen atoms add to opposite sides of the double bond) that led early investigators to postulate the existence of a halonium ion intermediate.

1H NMR spectroscopy worksheet

NMR-IR-SPECTROSCOPY-PROBLEMS-KEY

Chapter Twelve chemistry note

·         Alcohols and ethers are two classes of organic compounds that contain an oxygen (O) atom.

·         In an alcohol, the oxygen atom is part of a hydroxyl group (-OH).

·         In an ether, the oxygen atom is attached to two carbon atoms.

·         They both have structures similar to water.

·         One hydrogen atom of water is replaced by an alkyl group in an alcohol and by a benzene ring in a phenol.

·         Naming alcohols:

1.      Name the longest carbon chain containing the –OH group and replace the –e with –ol.

2.      Number the longest chain, starting at the end closest to the –OH group.

3.      Name and number the other substituents relative to the –OH group.

4.      Name a cyclic alcohol as a cycloalkanol.

5.      When the –OH group is attached to a benzene ring, it is named a phenol.

***The –OH is the first when counting.

·         Alcohols are classified by the number of carbon groups attached to the carbon atom bonded to the hydroxyl (-OH) group.

·         A primary alcohol has one alkyl group attached the carbon atom bonded to the –OH.

·         A secondary alcohol has two alkyl groups.

·         A tertiary has three alkyl groups.

·         Thiols are a family of sulfur containing organic compounds that have a thiol (-SH) group. It’s like alcohol except the –SH group replaces the –OH group. These are named by adding thiol to the longest carbon chain.

·         An ether contains an oxygen atom that is attached by single bonds to two carbon groups that are alkyls or aromatic rings. They have a bent structure like water and alcohols.

·         The first four alcohols are soluble in water.

·         Phenol is soluble in water because the hydroxyl group ionizes slightly as a weak acid. In fact, an early name for phenol was carbolic acid.

·         Alcohols burn with oxygen too.

·         During dehydration of alcohol, H- and –OH are removed from adjacent carbon atoms of the same alcohol to produce a water molecule. When alcohol is dehydrated, it turns into an alkene and water.

·         Oxidation means the loss of a hydrogen atom or the addition of oxygen. It occurs when there is an increase in the number of carbon-oxygen bonds.

·         The oxidation of a primary alcohol produces an aldehyde, which contains a double bond between the carbon and the oxygen.

·         Tertiary alcohols do not oxidize readily because there are no hydrogen atoms on the carbon bonded to the –OH group. C-C bonds are usually too strong to oxidize, tertiary alcohols resist oxidation.

·         In an aldehyde, the carbon of the carbonyl group is bonded to at least one hydrogen atom. The group name suffix changes to –al.

·         In a ketone, the carbonyl group is bonded to two alkyl groups or aromatic rings. The group name suffix changes to –one.

·         Naming aldehydes:

1.      Name the longest carbon chain containing the carbonyl group by replacing the –e to –al. No number is needed for the aldehyde because it’s always at the end.

2.      Name and number the substituents on the carbon chain by counting the carbonyl carbon as carbon 1.

·         Naming ketones:

1.      Name the longest carbon chain containing the carbonyl group by replacing the –e in the corresponding alkane name by –one.

2.      Number the main chain starting from the end nearest the carbonyl group.

3.      Name and number any substituents on the carbon chain.

4.      For cyclic ketones, the prefix cyclo is used in front of the ketone name.

·         Aldehydes oxidize readily to carboxylic acids.

·         Molecules are structural isomers when they have had the same molecular formula, but different bonding arrangements.

·         Another group of isomers called steroisomers have identical molecular formulas but are not structural isomers. The atoms are bonded in the same sequence, but differ in the way they are arranged in space.

·         Objects that have nonsuperimposable mirror images are chiral. Left and right shoes are chiral. So if the item is in the mirror it looks exactly the same only in the mirror.

·         A carbon compound is chiral if it has at least one carbon atom bonded to four different atoms or groups. This type of carbon atom is called a chiral carbon because there are two different ways that it can bond to four atoms or groups of atoms. ***Has to be four different bonds***

·         When steroisomers cannot be superimposed, they are called enantiomers. 

Chapter Eleven chemistry note

·         Alkenes and alkynes are families of hydrocarbons that contain double and triple bonds, respectively. They are called unsaturated carbons because they do not contain the maximum number of hydrogen atoms as do alkanes. They react with hydrogen gas to increase the number of hydrogen atoms to become alkanes, which are saturates hydrocarbons because they so have the maximum number of hydrogen atoms.

·         Alkenes contain one or more carbon-carbon double bonds formed when adjacent carbon atoms share two pairs of valence electrons.

·         In an alkyne, a triple bond forms when two carbon atoms share three pairs of valence electrons.

·         Naming alkenes and alkynes:

1.      Name the longest carbon chain that contains the double or triple bond. Replace the suffix with the respective new suffix, -ene or –yne.

2.      Number the longest chain from the end nearest the double or triple bond. Indicate using the lowest number. You have to name where the bond is.

3.      Give the location and name of each substituent in alphabetical order as a prefix to the alkene or alkyne name.

·         In alkenes, there is no rotation around the carbons in the double bond because it’s rigid.

·         A cis isomer, the hydrogen atoms are on the same side of the double bond.

·         Trans isomers have hydrogen atoms on opposite sides.

·         Cis or trans comes first when naming the group.

·         For alkenes and alkynes, the most characteristic reaction is the addition of atoms or groups of atoms to the carbons of the double or triple bond.

·         In a reaction called hydrogenation, two atoms of hydrogen atach to the carbons in a double bond of an alkene to form alkane.

·         In hydration, an alkene reacts with water (H-OH). The hydrogen atom forms a bond with one carbon atom in the double bond, and the oxygen atom in OH forms a bond with the other carbon.

·         When water adds to a double bond in which the carbon atoms are attached to a different number of H atoms, the H from HOH attaches to the carbon that already has the most H atoms.

·         Polymers are large molecules that consist of small repeating units called monomers.

·         Another hydrocarbon is benzene. Because many compounds containing benzene had a fragrant odor, the family of benzene compounds became known as aromatic compounds. A benzene molecule consists of a ring of six carbon atoms with one hydrogen atom attached to each carbon.

·         When a benzene ring is a substituent, it’s named as a phenyl group.

Chapter 10 chemistry note

·         Organic compounds always contain carbon (C), usually hydrogen (H), and may also have other non-metallic elements such as oxygen (O), sulfur (S), nitrogen (N) or chlorine (Cl).

·         It must have four bonds and they will always have an H₃ at the end.

·         Many organic compounds burn vigorously in air. Typically, organic compounds are not soluble in water.

·         Inorganic compounds are ionic, which leads to high melting and boiling points. Inorganic compounds that are ionic or polar covalent are usually soluble in water.

·         Most organic substances so not burn in air.

·         Hydrocarbons are organic compounds that consist only of carbon and hydrogen. The simplest hydrocarbon is methane, CH₄.

·         A molecule with four atoms bonded to a central atom has a tetrahedral shape.

·         More than 90% of the compounds in the world are organic compounds. The large number of carbon compounds is possible because the covalent bond between carbon atoms is very strong, allowing carbons to form long, stable chains.

·         The alkanes are a class of hydrocarbons in which they are connected by only single bonds. One of the most common uses is fuels.

·         Alkane prefixes:

1.       Methane     CH₄        

2.       Ethane          C₂H₆       CH₃-CH₃

3.       Propane       C₃H₈

4.       Butane         C₄H₁₀

5.       Pentane       C₅H₁₂

6.       Hexane        C₆H₁₄

7.       Heptane      C₇H₁₆

8.       Octane         C₈H₁₈

9.       Nonane        C₉H₂₀

10.   Decane         C₁₀H₂₂

·         In a condensed structural formula, each carbon atom and its attached hydrogen atoms are written as a group. A subscript indicates the number of hydrogen atoms bonded to each carbon atom.

·         The bonds can rotate freely about connecting the carbon atoms.

·         Hydrocarbons can also form cyclic structures called cycloalkanes, which have two fewer hydrogen atoms than the corresponding alkanes.

·         Cyclo always has two compounds less, the H₂ is connected on each side, it’s not a straight line. When it cyclizes, it loses 2H atoms.

·        

 

                  cyclopropane                         cyclobutane      cyclopentane       cyclohexane

·         When an alkane has four or more carbon atoms, the atoms can be arranged so that a side group called a branch or substituent is attached to a carbon. An alkane with at least one branch is called a branched alkane.

·         When two compounds have the same molecular formula but different arrangements of atoms, they are called isomers.

·         The carbon branch is named as an alkyl group, which is an alkane that is missing one hydrogen atom.

o   CH₃ = Methyl

o   CH₃-­- CH₂  = Ethyl

o   CH₃-- CH₂--CH₂ = Propyl

o   F--, Cl--, Br--, I--, Fluoro, chloro, bromo, iodo

 

o   CH₃—CH—CH₃ = Isopropyl

  

·         How to write the alkanes with substituents

1.       Write the name of the longest chain

2.       Number the carbon atoms starting from the end nearest a substituent (stay in that direction once you start numbering)

3.       Give the location and name of each substituent as a prefix to the alkane name

4.       If there are more than one substituent, call them di, tri, or tetra as appropriate

5.       No number is necessary for a compound with one or two carbon atoms and one substituent

·         To figure out if the chain is an isomer or the same molecule, count the carbons and hydrogens. If they’re the same, just rearranged, they’re isomers. If they’re the same but not rearranged, just flipped around, they’re the same molecule.

·         Many types of alkanes are the components of fuels that power our cars and oil that heats our homes.

·         Alkanes are nonpolar, which makes them insoluble in water. However, they are soluble in nonpolar solvents such as other alkanes.

·         The first four alkanes- methane, ethane, propane, and butane- are gases at room temperature and are widely used as heating fuels.

·         Alkanes having five to eight carbon atoms –pentane, hexane, heptanes, and octane- are liquids at room temperature and are highly volatile, which make them useful fuels such as gasoline.

·         Liquid alkanes with nine to seventeen carbon atoms have higher boiling points and are found in kerosene, diesel, and jet fuels.

·         An alkane undergoes combustion when it completely reacts with oxygen to produce carbon dioxide, water, and energy. Carbon-carbon single bonds are difficult to break, which makes alkanes the least reactive family of organic compounds. Alkanes burn readily in oxygen.

·         In organic compounds, carbon atoms are most likely to bond with hydrogen, oxygen, nitrogen, sulfur, and halogens.

·         Within this vast number of compounds, there are specific groups of atoms called functional groups that give compounds similar properties.

·         The alkenes contain one or more double bonds between carbon atoms.

·         Alkynes contain triple bonds.

·         Aromatic compounds contain benzene, a molecule that has a ring of six carbon atoms with one hydrogen atom attached to each carbon.

 

·         The characteristic functional group in alcohols is the hydroxyl (-OH) group bonded to a carbon atom.

·         In thiols, the functional group –SH is bonded to a carbon atom.

·         In ethers, the feature is an oxygen atom bonded to two carbon atoms.

·         Aldehydes and ketones contain a carbonyl group (C=O), which is a carbon with a double bond to oxygen.

·         In an aldehyde, the carbon atom of the carbonyl group is bonded to another carbon and one hydrogen atom. It’s always at the end.

·         In a ketone, the carbonyl group is bonded to two other carbon atoms.

·         In carboxylic acids, the functional group is the carboxyl group, which is a combination of the carbonyl and hydroxyl groups.

·         An ester is similar to carboxylic acid, except that the oxygen of the carboxyl group is attached to a carbon and not to hydrogen.

·         In amines, the central atom is a nitrogen atom. Amines are a derivative of ammonia, NH#, in which carbon atoms replace one, two, or three of the hydrogen atoms.

·         An amide, the hydroxyl group of a carboxylic acid is replaced by a nitrogen group.

Chapter 9 chemistry note

·         Isotopes are atoms of the same element that have different numbers if neutrons.

·         Most naturally occurring isotopes up to atomic number 19 have stable nuclei.

·         Elements with atomic numbers 20 and higher usually have one or more isotopes that have unstable nuclei, in which nuclear forces cannot offset the repulsions between the protons.

·         An unstable nucleus is radioactive, which means that it spontaneously emits small particles of energy, called radiation, to become more stable.

·         Radiation may take the form of particles such as alpha and beta particles, positrons, or pure energy such as gamma rays.

·         An alpha particle is identical to a helium (He) nucleus, which has 2 protons and 2 neutrons.

·         An alpha particle has a mass number of 4, an atomic number of 2, and a charge of 2+.

·         A beta particle is an electron that is emitted when a neutron in an unstable nucleus changes to a proton and electron. A beta particle has a charge of 1- and a mass number of 0.

·         A positron has a positive 1+ charge with a mass number of 0, which makes it similar to a beta particle.

·         Gamma rays are high energy radiation, released when an unstable nucleus under-goes a rearrangement of its particles to give a more stable, lower energy nucleus.

·         When radiation strikes molecules in its path, electrons may be knocked away, forming unstable ions.

·         The cells most sensitive to radiation are the ones undergoing rapid division- those of the bone marrow, skin, reproductive organs, and intestinal lining, as well as all cells of growing children.

·         Cancer cells are another example of rapidly dividing cells.

·         Alpha particles, the heaviest of the radiation particles, travel only a few centimeters in the air before they collide with air molecules, acquire electrons, and become helium atoms. A piece of paper, clothing, and our skin provide protection against alpha particles.

·         Beta particles move much faster and farther than alpha particles, traveling as much as several meters through the air. They can pass through paper and penetrate up to 4 to 5 mm into body tissue.

·         Gamma rays travel great distances through the air and pass through many materials, including body tissues. Because gamma rays can penetrate so deeply, exposure to these rays is extremely dangerous.

·         Symbols:

o   Alpha: α               Mass number:4                Charge: 2+

o   Beta: β                   Mass number: 0               Charge: 1-

o   Positron: β⁺                         Mass number: 0               Charge 1+

o   Gamma Ray: γ      Mass number: 0               Charge: 0

o   Proton: p               Mass number: 1               Charge: 1+

o   Neutron: n             Mass number: 1               Charge: 0

·         When radiation strikes molecules in its path, electrons may be knocked away, forming unstable ions.

·         The cells that are most sensitive to radiation are the ones undergoing rapid division: bone marrow, skin, reproductive organs, and intestinal lining, as well as all cells of growing children.

·         Alpha particles, the heaviest of the radiation particles, travel only a few centimeters in the air before they collide with air molecules, acquire electrons, and become helium atoms. Paper, clothing, and skin protect us from alpha particles. Dangerous if inhaled due to high ionization.

·         Beta particles move much faster and farther than alpha particles, traveling as much as several meters through the air. They can pass through paper and penetrate as far as 4-5 mm into the body tissue. External exposure can burn the surface of the skin. Use heavy clothing such as lab coats and gloves.

·         Gamma rays travel great distances through the air and pass through many materials including body tissues. Exposure is extremely hazardous, lead or concrete are required to stop them.

·         When a nucleus spontaneously breaks down by emitting radiation, the process is called radioactive decay.

o   Radioactive nucleus ànew nucleus + radiation (α β β+ γ)

§  It’s shown as a nuclear equation using the symbols for the radioactive nucleus, the new nucleus, the new nucleus, and the radiation emitted.

·         In a nuclear equation, the mass numbers and the atomic numbers must balance so the number of protons and neutrons are equal on both sides. It is often a change in the number of protons, which gives a different element.

·         An unstable nucleus undergoes alpha decay by emitting an alpha particle. Because an alpha particle consists of 2 protons and 2 neutrons, the mass number decreases by 4, and the atomic number decreases by 2.