Unit 16 - hydrocarbons - Distinction received PDF

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Kate Seeley – 06/10/2019

Hydrocarbons With carbon (C) being one of the most common elements on Earth, it is the basis for most everything. It can be combined with hydrogen (H) to create hydrocarbons, ranging in complexity from simple alkanes such as methane (CH4), to caffeine (C8H10N4O2) to alkenes and beyond, all having everyday uses to fuel homes, transport and industry. They’re all by-products of crude oil; a finite resource found in the Earth’s crust, remains of organisms which inhabited the planet millions of years ago (BBC Bitesize 1, p.1). It is a complex mixture of hydrogen and carbon atoms, and by refining can produce various fuel products on which humans rely. These include gases, petroleum, kerosene, diesel, paraffin, bitumen and oils for creating plastics, solvents and lubricants, and other petrochemicals. At present, humans are almost totally dependent on fossil fuels, which is linked to the issue of climate change as a result of combustion reactions releasing gases, such as methane and carbon dioxide (CO2) into the air. These gases are trapped within the atmosphere, reflected back to the Earth repeatedly, warming the surface further and negatively exacerbating the greenhouse effect. Without intervention the planet will continue to heat up, melting the ice sheets, raising sea levels, making weather Figure 1 - EarthHow more extreme, destroying farmland and affecting human health (Nunez, 2019). If we endeavour to replace fossil fuels with renewable energy sources, we can slow down the current rate of warming before it becomes irreversible. Alkanes are a family of single-bonded hydrocarbons. The family (appendix i) starts with methane, comprised of one carbon atom and four hydrogen atoms, followed by ethane (C 2H6). Following this pattern, the next alkane, propane, has the formula C 3H8. These are the simplest forms of the molecules as the amount atoms used are few. Isomers can have the same formula and content, but occupying different spaces on the carbon chain. Pentane (C5H12) can be rearranged into two additional chain isomers ; 2-methylbutane and 2,2dimethylpropane (figure 3). These all have the formula C5H12 but differ in terms of properties. When rearranged, the carbon atoms branch off from a carbon becoming known as a methyl group. Every identifying feature of an isomer contributes to the naming of it. Using the pentane isomers I drew (figure Figure 2 - Hardinger 3), the structure and names of each can be observed. 2-methylbutane is named as such because the carbon spine consists of four atoms. This is butane. The fifth carbon has become a methyl group above the second carbon atom.

Numbers prior to the chemical name simply indicate where a methyl group(s) can be found, hence 2,2-dimethylpropane; three carbon atom spine is propane. This introduces the term ‘dimethyl’, which dictates the molecule has two methyl groups and both are attached to the second atom (2,2).

Figure 1

Both alkanes and alkenes follow homologous series, which helps to determine their appearances and behaviours. Breaking the word homologous down describes exactly what it means: Greek: Homo –same, logos –proportion Together these words mean ‘consistent’, therefore the alkane homologous series follows a consistency using the same rule to advance molecular mass: CnH2n+2. The alkene rule is CnHn2. Being of the same series gives insight into common chemical and physical properties the family shares. The suffixes, –ane and –ene, establishes structural formula. Alkanes being single-bonded appear straight, in addition to methyl groups. The alkene series (appendix ii) starts with doublebonded carbon atoms, ethene (C2H4), and tend to have diagonal branches. In general, as molecular mass increases down the series, trends in melting and boiling points, as well as density also increase. The following graph is a visual representation of these trends. At room temperature, alkanes up to and including butane are gases, and pentane to decane (C10H22) are liquids. They start becoming solids at the larger molecule heptadecane (C17H36), which when compared to methane, is a lot less flammable. The appeal for using lower alkanes as fuel comes from their combustion reactions. Octane (C8H18) is used in petrol as, when it reacts with oxygen, it provides sufficient energy to propel a car.

Figure 4 - alevelchem

C8H18 + O2  CO2 + H2O As observed by this unbalanced equation, the products of this reaction are carbon dioxide and water, but the number of atoms don’t add up on both sides, i.e. it started with eight carbon atoms and the result has one, and they didn’t go anywhere. The correct volumes of carbon, hydrogen and oxygen are in fact:

2 C8H18 + 25 O2  16 CO2 + 18 H2O (Elroi Academy) When an alkane is introduced to a halogen (group 17 on the periodic table) and UV light, a hydrogen atom can be replaced, forming a haloalkane. Halogenation of an alkane is a substitution reaction, whereby a carbon/hydrogen bond is broken down and a new bond takes its place. The chlorination of methane can be seen in the following equation:

CH4 + Cl2 (+ UV energy) → CH3Cl + HCl In this case, methane has been broken down by presence of UV light and two chlorine atoms, known as free radicals. This is done in three stages: initiation, propagation and termination.   

The process is initiated when the free radicals are destabilised by UV, and want to quickly attach to a molecule willing to share its electrons. Propagation is reached when one of the chlorine atoms reacts with methane to create a methyl group, which then reacts with the second chlorine atom. The combination of atoms results in the end with CH3Cl + HCl or C2H6 + Cl2 reaching termination. These combinations can continue to rearrange after this cycle. Cracking is an important process for separation of the appropriate alkanes to be used in production of oil products. Fractional distillation of crude oil breaks apart heavy alkane molecules into lighter ones. For example:

C7H16  C5H12 + H2C=CH2 This can be done by using heat, pressure and catalysts. Figure 5 shows the process using temperatures of 350° to get oil to vaporise and condense according to temperature, feeding the separated products out. This also highlights those trends Figure 5 - Gowthaman molecules can have dependent on their size. Molecules collected from the top of the column are smaller, so they have low boiling points, high volatility, easily ignited and flows easily. The larger molecules at the bottom have high boiling points, low volatility, not ignited easily and don’t flow well. Alkanes are generally unreactive. They can be burned, destroying the whole molecule for energy and they can react with some halogens create haloalkanes, but not much else. This is because an alkane’s bonds between carbon and hydrogen atoms are stable and strong, which would require a lot of energy to break them. The second reason is due to the bonds’ low polarity. Carbon-carbon bonds have no polarity as they are the same atoms and carbon-hydrogens bond polarity is 0.4 on the Pauling’s scale. There isn’t enough electronegativity to attract other molecules.

Alkenes are another family of hydrocarbons which use a double bond. They follow naming rules shared by alkanes, but for one denoting the placement of that double bond. I have drawn four examples of butene. First two are but-1-ene and but-2-ene, where the bond is after the first and second carbons respectively (figure 6).

Figure 2

Images three and four are cis-trans isomers. Cis is Latin for ‘same’, and trans means ‘across’. Cis-but2-ene shows both hydrogen atoms on the same side and trans-but-2-ene has them opposing each other. These four molecules contain the same atoms but due to their arrangements can have differing properties. Alkenes all have a double bond placed somewhere in their carbon chain. Using a structural formula shows a simplified diagram of a molecule. Atoms are actually 3D operating space using X, Y, Z axis, as such electron shells have very specific shapes on which they orbit (appendix iii).

Figure a specific geometrical arrangement (English, 3.2).

Double bonds occur through orbital hybridisation, by mixing atomic orbitals to form new hybrid shells, when s and p orbitals combine. Hybridised carbon now has three new sp2 orbitals and a single p orbital (figure 7). This process combines energies to disperse them equally after hybridisation, and this forms

Hybridisation also creates a single sigma bond where the central sp2 orbitals overlap. This now inhibits rotation as they must stay parallel to each other. The last unhybridised p orbital bond to create a pi bond when they overlap. Carbon double bonds are more reactive than single bonds due to the pi bond which attracts electrophiles towards it. This is because of the electron placement within the pi bond. In the sigma bond, electrons are contained in the centralised overlapped area. The electrons of the pi bond have the entire area of their bean-shaped clouds. This also leaves them further away from both nuclei, weakening nuclear attraction. These factors make the pi bond extremely attractive to electrophiles, allowing them access to infiltrate double bonds. This is an addition reaction, two compounds come together to form one larger compound. Hydrogen bromide is an electrophilic compound because bromine is more electronegative than hydrogen. Hydrogen is slightly positive, it is attracted to the electrons in ethene’s pi bond. Hydrogen breaks the double bond to join on via a single bond to a carbon atom. At this point. The other carbon atom loses an electron to become a carbocation and the bromine an anion. As these have opposite charges, they bond to form bromoethane.

C2H4 + HBr  C2H5Br

Similar to the addition reaction above, bromine water (Br2) can be used to determine whether a substance is an alkane or an alkene. An alkene when placed in the brown water and shaken will break a double bond and insert their two bromine atoms to form 1,2-dibromoethane. This will be indicated by decolouring the solution. An alkane would remain brown. Polymerisation is another branch of addition reaction. By adding three ethene monomers together creates the smallest polymer, polyethene. These can be repeated to create longer polymer chains, by breaking double bonds using heat and pressure. The addition of chlorine atoms can create polychloroethene, or poly vinyl chloride, PVC, and adding a styrene makes polystyrene (BBC Bitesize 5). While plastics such as these are undoubtedly useful and somewhat durable, they bring us back to the beginning of this essay, in that their improper disposal is not so eco-friendly. Luckily people are beginning to see the damage caused by single-use plastics and are beginning to wean ourselves off of it.

Appendix 

Appendix i – alkane homologous series

Source: ePearl 2.1

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Appendix ii – alkene homologous series

Source: Thomas Tallis Science

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Appendix iii – electron orbitals

Source: Chegg Study

References list 

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Alevelchem.com, Alkanes [Online] Avaiable at: http://alevelchem.com/aqa_a_level_chemistry/unit3.1/sub316/01.htm (Accessed October 2019) BBC Bitesize 1, Crude Oil, hydrocarbons and alkanes [Online] Available at: https://www.bbc.co.uk/bitesize/guides/zshvw6f/revision/1 (Accessed September 2019) BBC Bitesize 5, Polymers, [Online] Available at: https://www.bbc.co.uk/bitesize/guides/zyfgmnb/revision/1 (Accessed October 2019) Chegg Study, Consider the s, p, d atomic orbitals [Online] Available at: https://www.chegg.com/homework-help/questions-and-answers/1-consider-s-p-d-atomicorbitals-illustrated--purposes-exercise-may-ignore-principle-quant-q13691889 (Accessed October 2019) Cotton, S., (2006), Ethene, aka Ethylene [Online] Available at: http://www.chm.bris.ac.uk/motm/ethene/etheneh.htm (Accessed October 2019) EarthHow, (2018), How the Greenhouse Effect Traps Heat and Warms Earth [Online] Available at: https://earthhow.com/greenhouse-effect/ (Accessed October 2019) Elroi Academy, (2015), Alkane Reaction Combustion [Online video] Available at: https://www.youtube.com/watch?v=7Rlm3K9580U (Accessed October 2019) English, S., Hydrocarbons [Online] Available at: https://www.epearl.co.uk/studymaterial/4582 (Accessed September 2019) FranklyChemistry, (2012), Electrophilc addition of HBr to ethene: an organic mechanism. [Online video] Available at: https://www.youtube.com/watch?v=Alb4d0FRHLY (Accessed October 2019) Gowthaman, N., (2017), Analysis of Various Liquid Components under Different Temperature and Density Constraints Pertaining To Fractional Distillation [Online] Available at: https://www.researchgate.net/publication/318053910_Analysis_of_Various_Liquid_Compon

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ents_under_Different_Temperature_and_Density_Constraints_Pertaining_To_Fractional_Dis tillation (Accessed October 2019) Grandinetti, P., (2019), Orbital hybridization [Online] Available at: http://www.grandinetti.org/orbital-hybridization (Accessed October 2019) Hardinger, S., Illustrated Glossary of Organic Chemistry [Online] Available at: http://www.chem.ucla.edu/~harding/IGOC/P/pentane.html (Accessed September 2019) Nunez, C., (2019), Carbon dioxide levels are at a record high. Here’s what you need to know., [Online] Available at: https://www.nationalgeographic.com/environment/globalwarming/greenhouse-gases/ (Accessed October 2019) Onwujiariri, T., (2017), Significance and Properties of the Homologous Series [Online] Available at: https://gulpmatrix.com/properties-homologous-series/ (Accessed October 2019) Thomas Tallis Science, Organic Chemistry [Online] Available at: https://sciencetallis.weebly.com/7-organic-chemistry.html# (Accessed October 2019)

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