Ch4.1 Basic Organic Concepts and Hydrocarbons Flashcards
What is a molecular formula?
Where the exact number of each atom in a molecule is shown
What is an empirical formula?
The simplest whole number ratio of atoms in a molecule
What is a structural formula?
Using conventional groups for minimal detail about unambiguous structure
What is a displayed formula?
Where the molecule is drawn with lines for bonds
What is a skeletal formula?
Shows a simplified organic formula by removing hydrogen atoms from alkyl chains to leave just carbon skeleton
What is a general formula?
Represents any member of a homologous series eg. alkanes or alkenes
What is the general formula for alcohols?
CnH2n+1OH
What is the general formula for an alkyl group?
CnH2n+1
What is an aliphatic hydrocarbon?
A compound containing carbon and hydrogen joined in straight chains, branched chains or non-aromatic rings
What is an alicyclic hydrocarbon?
An aliphatic compound arranged in non-aromatic rings with or without side chains
What is an aromatic hydrocarbon?
A compound containing a benzene ring
What does it mean if a hydrocarbon is saturated?
Contains single carbon-carbon bonds only
What does it mean if a hydrocarbon is unsaturated?
There is multiple double or triple carbon-carbon bonds or aromatic rings
What is a structural isomer?
Different name and structure, same molecular formula
What is a steroisomer?
Same chemical formula but a different arrangement of atoms in space
Define structural isomers
Isomers that have the same molecular formula but different structural formulae
What are the 2 types of covalent bond?
Sigma and Pi
Describe sigma bonds
The overlap of orbitals where the electrons in the overlap are located between the nuclei of the overlapping atoms
Describe Pi bonds
The overlap of orbitals where the electrons in the overlap are located above and below the nuclei of the overlapping atoms
Which is stronger out of a pi bond and a sigma bond?
Sigma bonds are stronger, in a reaction the Pi bonds will break first
How do Pi and Sigma bonds relate to single, double and triple covalent bonds?
single = 1 sigma
double = 1 sigma and 1 pi
triple = 1 sigma and 2 pi
How do boiling points differ between isomers?
- higher in straight chains than branched
- more branches = lower BP
- branches decrease effectiveness of intermolecular forces
- BP differs with different functional groups
Define stereoisomerism
Isomers that have the same structural formula but different arrangement of atoms in space
State 2 features of stereoisomers
- come in pairs
- the 2 isomers in the pair have the same name
Where is E/Z isomerism found?
In alkenes due to the restricted rotation caused by the C=C bonds
What is the condition needed for E/Z isomerism to occur?
Must be different atoms or groups attached to each side of the carbon atoms
What do the E and Z stand for in E/Z isomerism?
Entegen and Zusammen
Opposite and Together in German
What are the conditions for an E isomer?
- the highest priority atoms are across the double bond
- there isn’t an H alone on both C atoms in double bond
What are the conditions for a Z isomer?
- the highest priority atoms are on the same side of the double bond
- there isn’t an H alone on both C atoms in double bond
What are the conditions for a cis isomer?
- the highest priority atoms are on the same side of the double bond
- there is an H alone on both C atoms in double bond
What are the conditions for a trans isomer?
- the highest priority atoms are across the double bond
- there is an H alone on both C atoms in double bond
How is priority determined when deciding what kind of isomer a molecule is?
The higher the atomic number of the group attached to the Cs in the double bond, the higher the priority
What are the products of complete combustion?
Carbon dioxide and water
What are the products of incomplete combustion?
Carbon, carbon monoxide and water
What is heterolytic fission?
Unequal splitting of a covalent bond whereby ions are produced (pair of electrons tend to more electronegative element)
What is homolytic fission?
Equal splitting of a covalent bond whereby radicals are produced
What are radicals?
- species with one or more unpaired electrons
- formed by homolytic fission of covalent bonds
- formed during reaction between chlorine and methane
- formed during thermal cracking
What is the mechanism that is used in the chlorination of methane called?
Free radical substitution
What are the 3 steps called that are involved in free radical substitution for chlorination of methane and what happens to radicals during these steps?
- initiation: radicals created
- propagation: radicals used and regenerated
- termination: radicals removed
What is covalent bond fission?
When in the first stage of a chemical reaction, activation energy affects the covalent bonds by causing them to break
What is homolytic fission?
Occurs when a covalent bond breaks and each electron goes to a different bonded atom. Generates 2 radicals
What features do radicals have?
- very reactive
- neutral species
- have one or more unpaired e-
Define reaction mechanisms
Models that show the movement of electron pairs during a reaction
Define radicals
Species with one or more unpaired e-
Define homolytic fission
When each bonding atom receives one electron from the bonded pair
Define heterolytic fission
When one bonding atom receives both electrons from the bonded pair
Describe heteroltyic fission
When a covalent bond breaks and both electrons from the bond go to one of the bonded atoms. Results in a positive ion and a negative ion
Why are carbocations reactive?
They are very reactive intermediates because they are unstable
How is a covalent bond formed?
When 2 radicals or 2 oppositely charged ions collide they form a covalent bond
Define homologous series
A family of organic compounds that have the same functional group but successive members differ by CH2
Describe 4 features of alkanes
- saturated (only single bonds)
- have the general formula CnH2n+2
- have a gradation in physical properties
- similar chemical properties
What are the bond angles around a C atom in alkanes?
109.5
How do boiling points of alkanes increase?
They increase as the carbon chain length of the molecule increases because the surface area increases which increases the number of induced dipole dipole forces
How does branching affect boiling point?
More branching lowers surface area which decreases the amount of intermolecular forces so also decreases the amount of energy needed to break the forces
Why do alkanes have low reactivity with many reagents?
- large amount of energy needed to break bonds (high bond enthalpies)
- C-H sigma bonds have very low polarity
What are alkanes often used as and why?
Fuels because they transfer chemical energy into usable form such as thermal
What reaction is involved in combustion of alkanes?
A rapid oxidation reaction which combines oxygen with another substance
Describe the process of complete combustion
- in presence of plenty of air
- makes clean blue flame and transfers max thermal energy because it fully oxidises carbon and hydrogen
- only produces water and carbon dioxide
Describe the process of incomplete combustion
- limited supply of oxygen
- produces carbon, water, carbon dioxide and carbon monoxide
- makes a cooler dirty yellow flame
How are halogenated organic compounds formed?
Alkanes react with chlorine and bromine in the presence of UV light
Describe the halogenation process
- homolytic fission of halogen forms radicals
- H atom subbed for halogen atom
- only continues if conditions are maintained
Describe the 3 stages of the radical substitution mechanism
- initiation - formation of radicals
- propagation - 2 repeated steps that build up the desired product in a side reaction
- termination - 2 radicals collide and make a stable product
Why is radical substitution unpredictable and hard to control?
- reactions are randomly caused by a very reactive radical colliding with another species
- mixture of products are formed and they must be separated so they can be used by fractional distillation or chromatography
What are the conditions for chlorination of methane initiation too happen?
UV light or temp of around 300 degrees C
Does chlorination of methane have a high or low atom economy?
Low - there are lots of products but only one is desirable
State 5 features of alkenes
- they’re hydrocarbons
- they’re unsaturated, they contain at least one C=C bond
- their general formula is CnH2n (only non cyclic)
- they have a gradation in physical properties
- they have similar chemical properties
Describe the bonding in alkenes
- C-H bonds are sigma bonds
- C=C bonds are a sigmas bond and a pi bond (pi bond only ever formed after sigma bond and pi bond also breaks before sigma bond)
Why is there restricted rotation in alkenes?
- the pi bond restricts rotation around the double bond which makes alkenes planar in the region of the double bonds
What bond angle is there around a double bond in an alkene?
120 degrees
What are stereoisomers?
Compounds that have the same structural formula but different arrangements of atoms in space
When can steroisomers be formed?
If an alkene has different alkyl groups attached to both carbons in double bond then stereoisomers can be formed
Why can E/Z isomers be formed in compounds?
Because of the restricted rotation around the double carbon bond
When can cis/trans isomers be formed in compounds?
When there is an H atom attached to both carbons in the double bond
Are alkenes more or less reactive than alkanes?
More
How does mean bond enthalpy relate to alkene reactivity?
- the average energy required for one mole of a given bond to undergo homolytic fission in the gaseous state
What special features does a double bond have in an alkene?
- area of high electron density
- attracts electrophiles like Br2, HBr and NO2+
Define electrophile
An electron-pair acceptor. Examples include positive ions and molecules with a delta positive region of charge
What is an addition reaction?
2 or more molecules bonded to make one product
Describe how addition reactions occur in alkenes
- pi bond breaks and small molecule is added across the 2 C atoms that were in the double bond
- forms saturated organic product
What is the general equation for addition reactions in alkenes?
C2H4 + X-Y –> C2H4XY
What is hydrogenation and what condition must it occur in?
- addition reaction in which hydrogen is added across a C=C bond
- conditions: 150 degress celsius and catalyst
What is an example of a catalyst that could be used in hydrogenation?
Nickel
What is halogenation?
- when halogen is added across a C=C bond
Describe the products of halogenation
- alkene becomes saturated
- dihaloalkane is produced
What can halogenation be used as a test for?
Saturation
- when a halogen is mixed with a saturated compound there is no reaction
- when they are mixed with an unsaturated compound, there is an addition reaction
What occurs when an addition reaction with an alkene and a halogen happens?
Decolourisation
In an electrophilic addition reaction, what occurs when starting the reaction?
- Br2 is non-polar however C=C bond is rich e- area and induces dipole
- delta positive side of the molecule is attracted to the double bond
In an electrophilic addition reaction, what occurs during the reaction intermediates?
- e- from pi bond make bond with Br atom causing heterolytic fission of Br molecule
- positive charge remains on one C atom (forms carbocations which are very reactive)
In an electrophilic addition reaction, what occurs during the formation of 1,2-dibromopropane?
- 2 e- from the bromide ion are shared with the carbocation making a second bond and a more stable product
What is hydration?
- an addition reaction between a gaseous alkene and steam
- used in industry to make alcohols
What conditions are needed for hydration to occur?
- high temp (300 degrees C)
- high pressure (65atm)
- phosphoric acid catalyst
Describe the reaction of addition of hydrogen halides
- addition reaction in which a hydrogen halide is added across the C=C bond
- produces haloalkanes
- at least one H atom is replaced by a halogen atom
- hydrogen halides are gas at RTP so they are bubbled through the alkene to cause a reaction
What is Markownikoff’s rule?
‘In electrophilic addition to alkenes the major product is formed via the more stable carbocation’
eg. If there is a product with a tertiary carbocation and one with a primary carbocation, the tertiary one will be the major product
What is the order of stability in carbocations?
methyl is less stable than primary which is less stable than secondary which is less stable than tertiary
How would you write the name of a polymer of chloroethene?
Poly(chloroethene)
Define biodegradable
Affected by the action of microorganisms and environmental conditions, leading to decomposition
Define bioplastic
Material made from a renewable source that is biodegradable
How are biodegradable polymers created?
Sometimes biodegradable polymers (like plant starch) are mixed with an addition polymer (like poly(ethene))
- over time the plant starch breaks down and smaller polymer chains are made
- this leads to biodegrading
What is the concern about biodegradable polymers even though they break down into smaller polymer chains?
They smaller polymer chains can still do damage to the environment
When can a material be classified as a bioplastic?
When only the biodegradable polymer is used
What can plant starch be used to make?
Bin bags
When can a material be classified as carbon neutral?
If the material used to make them is derived from sustainable farming, the material can be classified as carbon neutral
What conditions must a material fall under for it to be classified as fully compostable?
- plastic must decompose as quickly as grass to compost
- only products should be water, carbon dioxide, inorganic ions and biomass
Describe poly(lactic acid)
- decomposes in around 180 days
- used to make cold drink cups and to wrap fruits and vegetables
How do photodegradable polymers break down?
- break down chemically using energy with wavelengths similar to light
- they are either addition polymers with bonds within their structure that are weakened by absorption of light OR they have an additive that’s affected by light
Is the break down of photodegradable polymers possible to stop?
No, once the polymer has been exposed to light, it begins to break down and the process is impossible to stop
What is the issue with photodegradable polymers in landfill?
They may not be exposed to sufficient light for them to degrade
